That many "Uber Drivers" borrow out their status to maybe a sibling, or friend who is possibly undocumented, that would not pass the Uber standards. Loan them their car...and Uber sign. (Undocumented citizens cannot get a driver's license), or possibly a friend who cannot pass a background check, maybe has a DUI....
But, may not pass an actual visual check on the picture ID.
Uber is making money when the car is going, so...they are probably not wanting to have a complete visual identification available.
Slippery slope.