One of the things preventing an Uber driver from just quitting is that Uber drives down price expecations across the board, and so makes it harder for others to pay their drivers a reasonable wage.

You might think that's ok, but in societies where we have agreed that having people go without healthcare or housing, or letting people starve is unacceptable, predatory employers end up in effect subsidised by the state, and those subsidies hurt both tax payers and more ethically run competitors.