1) On average the Uber car had to be intervened every 20 km or so. That's every 20-30min. If it happens that often, the additional driver would have to be doing something regularly during the day. Not looking so often at the road is dangerous. Uber should've been monitoring for this as well, just like they do with train drivers.

2) The train drivers in Europe are specifically hired to do just that. The ones in Netherlands are tested yearly. They're put on suspension when they run a red sign. Every so often they have to go through a special test for their attention span training. Uber obviously didn't do any of that.