They don't have to. Amazon has a sweetheart deal going on, there is no downside.

Amazon outsources the insurance, health, org, and most of the business risk to these sole supplier contractors. They can choke them to death and just say, "Welp, time to fund another one" until they get the results they want.

Only Amazon can determine which results are acceptable due to these arrangements.

That would make the drivers Amazon employees so they'd no longer be able to use the "it's not us, it's the delivery firms" defense.