Because its much better, and so attracts more people to the service.
Uber has a great potential to be used as public transport, with dynamic route pathing based on supply and demand. Something traditional public transport could never do.
Traditional public transport certainly could do that (and demand-response routes are a thing in rural places), but fixed-route consistent routes make more sense in urban areas from geometric considerations (and you know, not requiring a charged smart phone to get from point A to B).
Uber is a disaster as public transit. It's functionally little different than just having everyone drive themselves. Not needing parking is a big benefit, but it's still a bad idea.
Uber has a great potential to be used as public transport, with dynamic route pathing based on supply and demand. Something traditional public transport could never do.