Here in Portugal the vast majority of the drivers seem to be doing it full time (having 3 phones at the same time to use several apps and all that). Many (most?) of them are Brasilian immigrants. Uber is dirt-cheap too here, and gas is expensive, so I don't understand how they even manage to turn a reasonable profit. Taxis are significantly more expensive (easily twice as much in my experience).

Every market is different though, I can definitely believe that the situation could be very different in California.