Those below-prime numbers are being subsidized by the price of the car. The calculus isn't any different for a car dealership than it is for a consumer: if it made more sense to invest that money than loan it out with a car for collateral, the dealership/bank would have done the same thing. They give you low rates because they want you to buy the car at the price they ask.
In principle, you can pull out a checkbook and bargain back the hit you know they're willing to take on financing.
This type of consumer credit's also seems to exist to lure enough people other to other products of the cooperating bank. It doesn't need to be priced in into the product and could actually be paid for by the bank as lead generation. Gullible people might fall for the types of "Keep your monthly payment rate and afford a vacation" while they are predatorily increase the actual interest rate.
In principle, you can pull out a checkbook and bargain back the hit you know they're willing to take on financing.