I think in my area (Boston) if you looked at high-Uber ride zones you would find areas where it has been difficult for geographical or other reasons to build mass transit. The proposed Green Line extension is partly because of traffic value, and partly because acquiring the right-of-way is still possible. On the Red Line I ride to work, there may be some Uber spike because its near capacity at rush hour. Uber is a bad solution to this because it would only put more cars on Mass Ave, which is already at capacity. They're actually addressing this by buying redesigned train cars than can hold more people and make it through the stops slightly faster.

The technical term where people/animals want to walk is called a desire path. https://en.wikipedia.org/wiki/Desire_path