This is because traditionally, taxi drivers have gotten tips. The user can change the percentage to anything from zero to whatever. In addition, he can set it to a specific amount. If he wants to tip two dollars no matter the fare, be it twenty-two or fifty two dollars, he can do that.
Uber offers taxis in a few markets, only: Montreal, Toronto, Boston, New York, Chicago, Washington, San Francisco, Honolulu and Sydney, Australia. (At least those are the markets of which I am aware--I l earned about Honolulu only recently).
It must be kept in mind that what the TNCs have done is legitimise the illegal cab driver and encouraged more to join the ranks. In order to retain drivers, the TNC must have users. In order to induce people to forsake the legitimate, legal and licenced cabs for the newly legitimised illegal cabs, the TNCs must keep the cost as low as they can. It is similar to a reverse "charge what the traffic will bear". The TNCs keep the renumeration to the drivers as low as they think that the drivers will accept it. Another part of keeping down the cost to the user is elimination the tip.
Uber has taxis because it does perceive a persistent demand for them in some markets. It needs to have drivers for its users who want them, so it retains the tip. There are some who accuse Uber of using the taxis as a "gateway" to UberX, but, if that were the case, Uber Taxi would have vanished quite some time back. The best advertising always has been word-of-mouth. Uber is a household word, these days. In addition, Uber has become a noun, a verb and has taken on new adjectival and adverbial uses. Thus, there has been no need of a "gateway" service for some time. Marry, Uber wants the public to view it as a "one-stop transpo shoppe". "You want a ride, I got a ride. Here, try a limousine. Don't want to pay for a limousine? How about a cheap ride? Oh, you don't like unlicenced rides? How about a taxi? I have those. Got a lot of stuff or people? How about a van or SUV? Too expensive? How about a cheap van or SUV? I got it all. Tell me what you need or want and I'll send it to you." If you need to get somewhere on the ground, Uber wants to take you there.