The companies compete with each other for more 'routes' each day. They offer to take on more work, to do more. And then they hire anyone they can to fulfill those routes. The drivers themselves are on contracts with the small companies that can end at any time. Not making your rate? Up to the small company to decide if you have a contract tomorrow.
The contracts themselves presume the drivers will deliver a route in a specific number of hours for a fixed rate of pay. But newbies work at a much slower rate. And during the holiday season, they need a lot of extra labor- a lot of newbies.
Who is to blame? Amazon? The companies? The drivers? Capitalism gone mad? Take your pick.