Many states require the dealer to handle all titling transactions on the vehicles they sell as part of their licensing agreement. The rationale is the state will get it's sales tax money and the title transfer will take place in a timely manner protecting the seller, any lienholder, the toll authorities etc.
If the dealer just handed someone the title, they could conceivably drive the rig forever without actually transferring ownership from the previous owner and never pay any sales tax. That would create a huge problem for the seller since they would be the owner of record when the driver runs a red light camera or crashes into a bus full of personal injury lawyers. Yes, I know this could happen just as easy with a private sale, but the vast majority of sales are conducted by dealers, so the potential for problems is at least diminished.
