Because their job is to sell you someone else’s problem. While you can buy a used/previously owned vehicle that doesn’t have problems, if it does, they still have to sell it, and as long as it lasts longer than your local “lemon law” (if you have one) says, then it’s your problem after that and you have no recourse other than to sell it and still lose a chunk of money. (Most likely, after they sell it, and after the lemon law makes it entirely your problem, they spread the word around that the car is, in fact, a lemon, so other used car dealers won’t touch it.)
Why not new car salespeople?
Because their job is to sell you someone else’s problem. While you can buy a used/previously owned vehicle that doesn’t have problems, if it does, they still have to sell it, and as long as it lasts longer than your local “lemon law” (if you have one) says, then it’s your problem after that and you have no recourse other than to sell it and still lose a chunk of money. (Most likely, after they sell it, and after the lemon law makes it entirely your problem, they spread the word around that the car is, in fact, a lemon, so other used car dealers won’t touch it.)