> You can only do things that some planning board sets up a route to get you from A to B

No, I have an entire grid of transit options to go most places within the city. It's not just A to B, it's anywhere along the co-ordinate grid formed by all the transit routes, plus what I'm willing to walk or bike. Moreover, it provides far easier access to most of the places I do want to go than driving would.

With a car, A to B is possible given that (1) there's car parking at both ends, (2) there isn't overwhelming traffic in-between those two points, and (3) that anywhere else you might want to go also meets those criteria. Providing (or guatunteeing) (1) (2) and (3) results in massive expanses of parking lots which are rarely ever filled, massive expanses of stroad to support all the traffic trying to get to those parking lots, and pushes everything further apart just to support cars, all requiring maintenance and upkeep. Not that trains don't, of course they do, but they're far more efficient in terms of space and money usage to person-miles travelled.