Couple reasons. Main one being the same reason you see so many Montana plates in comifornia. Secondly, I’d prefer to talk to the person who actually owns the vehicle and been driving it if I’m spending 80 thousand dollars. Not some scum bag salesman who knows nothing. Then sit with the finance guy who tries to sell me snake oil for an hour cause that’s how he makes a commission.
As for the warranty, a warranty and the sales end of a dealership have nothing to do with each other. I have a good relationship with my local ram dealer so if I can get something under warranty even for a few thousand miles I can assure myself a better success rate at weeding out any issues that aren’t easily apparent at purchase.