You say this as if it’s cheap or easy to make any kind of significant public transit improvement in an already established city.
Public transit is hard is in the same way that going to Harvard as a black person in 1900 was hard. It was hard because malicious forces (whether they accepted the accusation or not) made it impossible.
Painting a BRT line and some traffic cones is all you need to set up public transit. Hell, almost all pre-established cities had entire street car networks that we disassembled due to car-centric post-1950s development.
Here's SF, Seattle and LA pre-1950.
https://i0.wp.com/transitmap.net/wp-content/uploads/2013/04/...
http://www.tundria.com/trams/USA/SanFrancisco-1944.shtml
https://la.curbed.com/2015/11/9/9902244/red-car-map-los-ange...