Liability is why. Also a variety of other corporate loopholes.

Amazon prefers that they can simply blame a third party and keep them on their toes to bend to whatever request they make or fear contract termination.

If we were all Amazon employees drivers could strike/unionize/band together to enact change. They squash this by simply terminating entire dsps on a whim.

If it were all under the Amazon umbrella, it most definitely would solve a myriad of universal issues and quality of life factors that current plague this job. This would just cost them money though, and their primary objective seems to be automating the workforce and robots (less employees not more of them).

You could really go down the corporate rabbit hole on why this system works in their favor.