I've had my oil changed numerous times at half a dozen different places over the years. I've done it my self more times than that.
I've never encountered anything like what you describe. I am sitting right in the car while they're working, so I can sort of watch what they're doing, and I do make a point of checking things out before getting very far away from the shop.
Further, a number of my friends (at least when I was younger) worked at places like this and, while many of them did smoke pot, I can't recall any of them laughing about intentionally damaging someone's car. They were pretty closely supervised, and smoking weed and even dropping out of school, while not great ideas, don't make someone an asshole.
So, uhm, anecdote right back at you.
Karma can be a bitch, and payback for the "that kind of stuff never happens to me" attitude, can be ugly. I know it's anecdotes, but F'ups are amazingly prevalent. After an oil change, my mom left the shop of a guy who was, at the time, was a trusted friend. Within blocks the oil plug fell out, and by the time she saw the idiot light and pulled over, the motor seized. Naturally, the culprit failed to man up, but the small claims judge did it for him. A neighbor pulled into a local place for an oil change, and was called into the office to be told very sternly, that her front struts were so shot that the car shouldn't even leave the garage. She called her husband, who informed her that he replaced them less than two months ago. I made the mistake of taking our CRV to a dealer while on the road, for an oil change. They had no interest in doing the work, they only wanted to get the car up on the lift to look for things to upsell. They tore the glove box apart to get to the cabin air filter. They then reported that it was "shot", which was total bullshit. I told them to put it back and do the work I asked them to do. They put the old filter back in, upside down, and failed to reassemble the interior of the glove box. After the oil change I had to use a cake pan, and while laying in the driveway of a resort, drain a extra quart of oil out. Why should a Honda dealer have any idea how much oil a new Honda takes, Right? My son gets serious with his fiancé , and tells her that it's time to stop taking her Corolla to the local quick lube clowns. The car has a canister style filter. He opens the top and discovers an odd lump in the bottom of the canister. The quick change place never changed the filter, and it had literally disintegrated. It hadn't had a new filter in years, even though she paid for almost a dozen.
Sure, the vast majority of service work gets done well enough to keep the vehicle running, but if you have the skill set and tools, IMHO, there is no reason to risk a valuable asset to a bunch of clowns who may, or may not, do the job correctly. If you need to sit in the car, watch what they are doing, and then double check their work before you get too far away from the place, you need to step back and ask yourself "why an I doing this, when it would be faster, easier, cheaper and done correctly the first time, if I did it myself"
I have done hundreds of oil changes, repairs and maintenance tasks on vehicles in my garage, and never had problems that were caused by not giving a shit enough to do it right the first time. In that time I have saved hundreds of hours by not driving back and forth to shops, and waiting for work to be done, and thousands, or more likely tens of thousands of dollars, by not paying a shop for labor, profit and overhead.