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.