Ask HN: Why doesn't Uber pay drivers using free market bidding?

There seems to be a lot of debate on things regarding the amount which companies like Uber and Lyft pay their drivers, whether surge pricing is ethical, etc. But all of this stems from the fact that the company directly sets the price of the service.

To me, it seems like it would make more sense to let drivers set their own price per mile / minute and to have to service automatically select the cheapest nearby driver. That would also give the user the ability to request a higher rated driver or quicker pickup (and pay more) or a lower rated driver or longer wait time (and pay less).

Why haven't they gone this path? Are there ecomnomic considerations in not thinking of? Is it just a matter of being too complex to implement?