Because most public transportation is less efficient than autonomous cars.
The exceptions are very high bandwidth routes carrying a consistently high amount of riders.
Most of the time for most routes you have large, expensive, low gas mileage vehicles running mostly at a loss.
Autonomous cars will be able to work efficiently in a dense configuration where they can operate very close together achieving almost the equivalent of the best of mass transportation.
And for the rest of the time (probably > two thirds) they are simply more efficient than pretty much any other alternative.
Autonomous cars also optimize for peoples time. The latency of your travel (time to get from a to b) will be lower. Since you don't have to walk to the closest bus stop, then take the local feeder bus to mass transit, then switch and wait for that, etc. You get picked up at home, and read your paper or work on your laptop until it drops you off at work.
Finally, building this out doesn't require mega-investments by local or state or federal governments. Since autonomous vehicles will re-use the existing road network (and more efficiently at that) multi-billion dollar investments in public transit lines won't be needed. Just encourage adoption of autonomous vehicles. As the old fleet ages out and the new one rolls out it becomes more efficient in its use of the road network.