BMW, Mercedes, Audi - what’s going on?

I’m in the process of researching to buy a used car from a private seller and I noticed that a good chunk of the cars I see land in those 3 luxury car brands. Does anyone have any idea why that’s the case?

I want to consider a luxury car if it’s in my budget, but it seems like these owners know something I don’t.

Also, what car/truck/suv/etc would you recommend to someone that has a $6000 budget?