Because on average you'll pay the same in repairs or replacement compared to a more recent car. You got lucky, and that's ok. But the prices aren't completely made up. Your BMW costs a lot less, because the expected average time it will survive is low. Anecdote != data and all that.

On the other hand, having an ex-demo car from a dealer meant no silly markup and when the transmission fell apart after 4 years, I got it replaced for free instead of paying thousands to an independent shop. There are pros and cons.

(And yes, completely new cars have silly markup, but used ones with low mileage don't)