I have been told several different things.
One eye doctor told me that most contacts are the same or very similar material. Therefore, the dailies, weeklys, monthlies, and annuals are pretty much the same. I can see this as being pretty possible. My yearlies kept their shape longer than the newer disposables, but they feel pretty much the same.
One told me the week/ month time period starts from opening the package and exposure, so even if you wear them once a week, they still only last 2 weeks. Like a chemical reaction? This seems unlikely, but possible.
I've also been told the change is related to how people care for contacts. Back when the majority were on yearly contacts, there was no "no rub" solution. You had to rub every day, both sides, 10 seconds, and use "extra" cleaner once a month or something. Then they realized people didn't do that, got infections, etc. So they "invented" dailies etc. and "no rub." Now, you could use this "no rub" which was pretty much the same, on your "monthlies" which were pretty much the same and you wouldn't get an infection, because you changed every month! They found some people were wearing them over night, so they made them slightly more air soluable, and sold them as over night for weekly!
I don't know which doctor was telling the truth. I try to rub them at least once a week, and I wear them until they bother me. Usually that is like 5-7 weeks. During allergy season, it is 2 weeks. I never wear them overnight.