I’ve never used Uber but know lots of people who like the ease of using its app to book and pay for services. In my opinion, businesses should provide options to its customers. In this case, paying through cash could be an added feature that may attract other users (that segment of population who still carry cash and actually prefer transacting with it).
For example, I usually use my card to pay for my groceries; but there are times when I would pay in cash so I can get my change in coins which I would then use to pay for parking. The point here is that I appreciate the fact that I am able to choose which payment method works for me.
It may well be additional work for Uber providers but if they want to reach more users, I believe providing more options (in booking and payment) should not be overlooked.