This question was based on wrong assumption. From experience I now know that Uber is cheaper then a regular taxi. Rome2Rio is predicting the wrong prices. Can't delete this question, because it got answers, hence this disclaimer.
I have been using Uber on two occasions where I became quite a fan. Not only because of the ease of usage, but also because of the very interesting fees (cheap). When I arrived in Rome I noticed that Rome2Rio now includes Uber in their proposed routes. However, Uber seems to be more expensive then a regular taxi. Why is that?