I think people are overthinking this and the whole issue is ridiculous. Uber doesn't expect you to tip and doesn't offer the feature, so I don't. If I take a cab I do and a Lyft I might tip.
Tipping in cash just reduces the pressure that Uber drivers can put on Uber to make sure the rev share is appropriate rather than exploitative.
i once tipped an Uber to the airport with extra cash when we had five or six cases and a baby and I was generous. But that's the exception.