Because then they can charge more.
I'm sure you guys know this. When a car company advertises 0% interest for however many years the interest of that period of time is built into the price of the car already. Sounds like a great deal but they still have you bent over the desk.
So if you buy the car with cash Like I always do keep in mind to calculate the amount of interest included in the price and deduct it off the asking price. Have to outsmart these crooks.