Ever notice that you don't see adults on BMX bikes? That's probably because you can only ride around with your A$$ lower than your knees for so many years before the pain starts.
Seriously, though, I think it is just ignorance. I know I rode around with my bike seat a couple inches low for years. It wasn't until I tried more demanding things that the pain and low performance drove me to figure out the correct seat height. I'm talking about trying to keep up with a group of experienced roadies for a few hours or riding with a fixed gear cog. Those endeavors will tell you if your seat is the right height. Just cruising around town or riding 5 miles to work can be done quite easily with the seat too low.