[nroets]

There is also another reason: If Uber employs the driver and he/she acts unprofessionally, then it's a lot of work to fire him/her before they can hire someone else. (Uber does not want to drag passengers into any type of litigation)

If Uber deals with many companies, they can just reduce demand at the company with the bad driver and increase demand at another one. The company with the bad driver can then retrench him, saying they need to cut someone.

(Disclaimer, I know very little about Uber and the US and it may not work for them specifically. But the idea certainly applies to other companies)