Because as much as trains and buses are great for everyday commuter movement (and having amenities within walking distance is key as well), there’s two issues:

• Changing the infrastructure and zoning of an existing city is much easier said than done. Ripping up concrete, tearing down existing business and homes to increase densification, that’s a huge undertaking.
• Trains never replaced the horse drawn carriage. You can never fully eliminate the need for cars because sometimes you need to move something big like a couch. Even if there’s less cars on the road, it’ll never be 0, as this also includes things like ambulances, and fire trucks that can’t rely on schedules.

I would just settle for sidewalks, where I live in the USA it’s just streets with no sidewalks everywhere. I used to live one mile from where I worked and I could barely even bike there because of crazy car drivers and nowhere to go if someone wasn’t paying attention. Rural America is going to be car dependent for a long, long time.

I dunno what country you are from, but here in the US of A, the monopolies that own all the train infrastructure make sure to keep trains as public transportation as cost prohibitive as possible.