A road network has a much lower initial investment cost and can just sit there even if it's underutilized, accruing comparatively very little in terms of ongoing maintenance costs, with no one needing to hyperventilate constantly about running a passenger service with enough asses in seats with sufficient regularity to recoup the costs. It can also go to a lot more destinations, and importantly can do so for mixed uses.

You could build train rails to the moon and back if you felt like it, but they'd decay quickly and it is statistically certain nobody would be riding on them to most places to justify paying for the upkeep, let alone the initial installation. This is not so with rural roads. There are also massive planning and logistical challenges you need to take into account for rail, mostly due to the fact that trains are crap at climbing inclines which is not an issue you'll face (as much) with roads. This is why roadgoing vehicles have tires (well, one reason). You'll be flattening mountaintops and boring tunnels and building bridges all over the place. It's not cost or environmentally effective for little-utilized routes.

No matter how well the mass transport system gets, you'll still have to have the road network. Every business will have to get stuff delivered by large truck. Every housing area will have to get streets, and you also can't eliminate highways. You could only potentially reduce how wide they are in some areas.