It's not possible to build good public transportation after people have already chosen where to live, work, and build buildings based on the non-public-transit infrastructure.
To use an Americentric, and worse, SV-centric example, consider when SF's BART was built. They trenched Market Street through almost all of downtown! Building transit even in dense urban areas is clearly doable with enough political will.
Maybe I'm doing the typical HN thing and assuming that lots of people are like me, but I've been moving around the SF bay area, and being close to transit is a massive consideration.