Yes, in an ideal world, they would estimate the cab price. But in an ideal world, iPods would come preinstalled with music and plane tickets could be purchased 5 mins before the flight at the same price. But these things just don't make business sense. I have a strong hunch that the time and effort put in by Uber to predict prices would only marginally increase their revenue, and certainly wouldn't increase their profits.
I was under the impression that this was not the case with Uber- that they specifically reached out to black cab companies to partner with them and establish rates. I may be wrong, but it's a real shame if I am.