> Why do people borrow money to buy a car?

Because new cars are nice, and people are bad at time value of money? Also I think it's partially a cultural thing, very few of my friends and family buy brand new cars but I know some people would never even consider a second-hand vehicle.

> It's almost as bad as cheating on your wife (and far, far worse than cheating on a gf).

What a curious point for comparison! Although I get your point if you just mean purely financially.