In Australia we've tried tipping Uber drivers (e.g. because we've inconvenienced them with a stop to pickup food on the way home) and so far after 5 occasions no driver has accepted the tip (taxi drivers accept a tip without question, and usually won't accommodate short trips, despite the law, yet alone special requests).
The whole thing seems to illustrate just how far out of whack tipping expectations are in the US compared to the rest of the world.