^ Excellent discussion above. The old rebuttal would be that curiosity is inseparable from intelligence. Therefore if intelligence evolves, it will become curious, and eventually that curiosity will drive intelligent life into space.
However, we humans are not curious about all things. Our curiosity is somewhat directed by our values, beliefs, culture, evolved traits, and body configurations. I can't help but to wonder if civilizations on earth evolved from dolphins, whether those dolphins would be more interested in exploring the deep sea rather than space. For humans, space is more interesting because it allows our primary sensory organ - vision in a spectrum of light - to see extreme distances. Dolphins might find the deep sea more interesting because they have sonar. Intelligent plants might be primarily interested in meteorology or geology. The attention-starved astronomers in an intelligent civilization of dolphins or trees might only be consulted or heard from when there is an issue, the way we treat meteorologists or oceanographers. Or simply, what if intelligent ET civilizations must meet their needs in ways that cannot be industrialized - for example, having to graze or filter food 90% of their waking hours? That might mean there's insufficient opportunity for tech development or education.
Also, much of humanity's space technology was developed to fight wars with one another. If humans had maintained a state of world peace, how many centuries or millennia would have been required to develop the technologies of the 20th century? And if we're not constantly interested in lobbing projectiles and explosives at each other, at what point does the aerospace information learned generations ago get lost to history faster than it can be reproduced? Humans fight wars because our bodily configurations, evolutionary history, sexual habits, and environmental needs favor raiding, competitor destruction, and resource acquisition behaviors. But other forms of intelligent life might not have the competitive/aggressive drives of humans - intelligent ocean plants or hive species for example. Other intelligent species might be too competitive/aggressive to cooperate and escape the Hobbesian state of nature - and that means no space program for them too.
Maybe they just evolve in a direction that doesn't involve language or prehensile appendages. Or maybe their lifespans are too short for their scientists to perform for very long at their peak. Or maybe their planet is permanently covered with clouds, and so they never even wonder about the stars they can't see - just like the opacity and darkness of the ocean and ground keeps most humans from wondering what lives in the deep sea or the soil. There are lots of ways for intelligent life to evolve and be unable, unwilling, or uninterested.
Intelligent aquatic animals or plants would also face the space exploration barriers of how to lift into space the heavy water or soil they need to immerse themselves in, whereas we primates mostly require lightweight air to immerse ourselves in. These weight requirements might make rockets an engineering impossibility for their species. Similarly, the math might not work out for species developing kerosene rockets on higher-gravity planets, species with metabolic needs that require them to pack more air, water, or solids than humans, or species whose planets lack fossil fuels.
The
space shuttles could lift 2,270kg to geostationary orbit, but their takeoff weight was 2,030,000kg so the payload represented 0.1% of the total weight of vehicle, load, and fuel. And that's to geostationary orbit, not launching things into deep space! Had earth's gravity been 0.1% greater, this particular spacecraft design might have never been able to put things into orbit. Maybe other designs could work on slightly more massive exoplanets, but I think this example illustrates the tightness of the engineering tolerances for civilizations starting out in space travel. And if a civilization cannot possibly start a rocket-based space program, do they just give up on space altogether before developing next-generation technologies? Maybe.
Out of the
1,400 star systems within 50 light years of Earth, let's very generously assume intelligent species could possibly evolve on 1/200 star systems, or exactly seven of them. For some planet or moon in that star system, the radiation is right, the gravity is right, the temperature, gas pressure, and environmental toxicity are right, etc. The planet or moon and its system stays geologically and temperature stable with flowing water and weather cycles for millions of years at a time.
Let's also generously assume the intelligent civilizations which formed over the past billion years can exist for an average 200k years (by comparison, we're <5k years from Babylon, so this is a very generous guess). Let's also generously assume intelligent civilization eventually evolves in 100% of circumstances where they could evolve. Under these assumptions, there is a (200k/1B=) 0.02% chance any particular intelligent civilization that occurred is existing at this (or any other) time. So out of the seven habitable systems within 50 light-years, you have a 1.4% chance of coexisting with at least one of them at the same time.
Let's assume it takes at least 5k years, as it did with humans, for a species to evolve from civilization-building to space-traveling and wireless communication and let's assume they continue these behaviors until the day they suddenly go extinct, rather than declining or evolving away their intelligence for the last few tens of thousands of years of the civilization's existence. This leaves them with 195k years to develop technologies which break what we consider to be the laws of physics (e.g. faster than light travel) and make physical contact with our civilization. This lowers the 1.4% odds negligibly. I won't bother with the math.
Then let's reduce that number by a guessed percentage of the number of civilizations that for whatever reason (resources like fossil fuels and aluminum, traits of their bodies or psychology, traits of the planet such as gravity or tidal lock, peacefulness/hostility, interest/desire) do not develop space programs. An 80% reduction sounds like a good WAG to me because there are so many good reasons. Now we're down to 0.28% odds of being within 50 light years of a space-faring ET.
Now, out of that 0.28% chance, let's assess the probability that our ETs actually figure out how to break the known laws of physics and travel faster than light and then decelerate without dying, figure out how to shield their spacecraft from cosmic and solar radiation, and hover in gravitational environments without ejecting mass. If there is no possible physics-compatible solution to ALL of these issues, there can be no UFO's flying around the countryside. Of course, they may have thousands of years to work on these problems, and humanity broke a lot of the old assumptions over the past 150 years, so this is the hardest factor to assess. A human from just a few generations ago could not have possibly imagined where science and culture would take us, so guessing what problems an intelligent civilization can solve is a humbling task. Let's split the difference and say the odds are 50/50 that a motivated and able alien civilization could, after a few thousand years of work, figure out how to violate what we know as the laws of physics.
So now our odds of UFOs are 0.14%. However these are the odds of alien civilizations within 50 light years possessing the required spacecraft at this time, not the odds that they visited earth or that the people who claim they saw UFO's actually saw one of these spacecraft. I.e. people would say they saw UFOs and strange things would appear on cameras and radars whether we were being visited by ETs or not. There is a 100% chance of some false electronic signals, misunderstood signals/sightings, manifestations of mental illness, spontaneous hallucinations in otherwise sane people, and outright frauds. What we know about humans is they make up stuff, and what we know about cameras is that the images can be deceiving.
Actually visiting earth, even from a star system <50 light years away might take an ET civilization hundreds of years to accomplish once they decided to do it, or it might be so costly in terms of the labor, energy, and resources required to launch the expedition that exploration is done extremely slowly. Consider that we choose to spend only a fraction of 1% of GDP on space exploration and R&D and that we haven't yet bothered to fully explore most of the ecosystems in our own oceans or rainforests even though we have the capacity to do so. Note that the ETs might also have 1,400 star systems to explore within 50 light years of themselves and only a couple hundred thousand years to do it. Our system might not have made the prioritization list if it's not easy to get to or doesn't look like their star system. Maybe they are desperately seeking a new planet which looks like theirs, and Earth is not it - too much gravity to hold up those light-bulb heads or whatever. So let's say there's a 50/50 chance that if an ET civilization with UFO technology exists within 50 light years of us, they are interested (and reckless?) enough to send UFOs to fly around in our planet's atmosphere in plain view of a few people. So let's reduce our 0.14% chance of UFO existence to a 0.07% chance of UFO sightings being real and leave it at that.
