Why do car dealerships offer voluntary repos if you still have to pay?

The "voluntary repossession" allows the purchaser who has defaulted the loan to avoid having to pay a repossession fee of a few to several hundred dollars. That is the ONLY advantage to the purchaser.

The dealership has the vehicle back in it's possession and can unload it at an auction without having to pay a repossession fee to a repossession agent (because the purchaser "voluntarily" surrendered the vehicle).

Once a car is repossessed, voluntary or not, it is (at least in most states) sent to an auction to be sold and the proceeds applied to the outstanding debt of the borrower.

Usually, there are specialized auction houses dealing solely, or nearly solely with repossessed vehicles and the vehicles NEVER (that I'm aware of) bring anywhere close to the amount of the loan.

This is true because first, it is (1) a dealer only wholesale auction, (2) the vehicle is a "repo" and (3) the "retail" price previously paid by the purchaser/borrower is an amount greatly inflated over and above the actual "wholesale value" of the vehicle.