I drive Uber from time to time. I've only gotten a tip once over ~50 rides.
A lot of the time the traffic conditions mean that after costs (I assume $0.40 per mile costs for mileage, gas, maintenance, etc.) my hourly is closer to $8 an hour (because after I drive for a while sometimes I need to drive a long way back as well on my own dime), which is below minimum wage. That's not including having to pick up plastic bottles after people or cleaning seats so they don't smell like sweat.
It's basically never worth to pick up anyone who is going far away from your house, you end up actually driving for free since you have to drive there and EVENTUALLY drive home. Why do you think Uber drivers don't want to drive 40 minutes away just to get paid less than if they drove for 40 minutes inside their own city? I'll never accept a ride to San Francisco (40 minutes away) ever again, it's never worth the price unless there's surge pricing in effect.
If I make $8-12 an hour, getting a $1 tip is like a 10% increase in my wage. I don't badger people for tips, but if I got like 10% of people to tip me, it would be super worth. The one tip I got was $20, btw. Even if we assume a 12% tip rate up from 2% if I ask for them explicitly, and the average tip size of $5 (median would be more like $2, but if I get a few more $20 tips the average would be higher than the median), I would make $0.50 more per hour which is a 5% increase in my earnings.
Maybe it's something I should actually start doing.