I think you're right to a point. During the period Uber and Lyft pulled out of Austin in protest of its background check ordinance, several other companies sprung up immediately and appeared to thrive until the big boys came back.

The barrier to entry is actually incredibly low, assuming you have a base of people willing to drive already as happened in Austin. You don't need to employ drivers or own cars, or anything else in the physical world that eats up capital. It's literally only an app.