Uber wanted to make payment so easy you don't even have to think about it. Tipping seriously gets in the way of that ideal, even if it's purely digital.
The first few times I used Uber, the most impressive aspect of it to me was that at the end I could literally just get out of the car. It was done. Payment was already handled. This was a tremendous improvement over the experience with taxis, where paying could be stressful and awkward and not even easy.
To this day I'm strongly against tips for Uber. Uber already has a rating system. I don't need the ability to decline to tip to communicate that a driver did poorly: I can just give them a low rating. So the only argument remaining for tipping built into the app is to supplement their pay, but that is a slippery slope that will quickly hurt drivers.
As soon as drivers routinely expect and rely on tips, that opens up Uber to reducing their rates to grab more customers. Drivers will become totally reliant on tips to make a decent wage, just like servers are in the restaurant industry. But not every customer will tip appropriately, which means one of two things:
(1) Either the drivers' overall compensation will go down due to the non-tippers, or
(2) The expected tips will go up, so that tipping customers effectively subsidize non-tipping customers.