Do people not remember the economy can go in the toilet really fast? Why aren't people constantly scared and thus saving as much as possible and acquiring good assets? Even if the economy stays strong, your own career or profession can go bad or get disrupted real fast.
As far as why people buy high end cars when they're not very rich, going into significant debt, I suspect it's because a car is a status symbol for so many people. If I could do without a car, I would. I'd be saving about $200 a month on my current expenses if I didn't need one - it's just a crappy old Mazda wagon, certainly no status symbol there.
The unfortunate reality is that (most) subprime borrowers don't have the luxury to consider catastrophic events that can occur in the future because they are too busy worried about today.