Because in our current state with everything built around cars, creating a train system to accommodate it all is nearly impossible. Trains work great in a downtown, or centralized area. They are very difficult to build to accommodate our 1x1 grid system that cars use. Or at least that's my perception of it. Even if the system could be built, it'd have to be manned, it'd have to travel to certain areas at certain times to account for jobs. And it becomes increasingly unwieldy the more requirements you add to it. I wish things had built up along a sustainable train systems instead of cars, but placing a train system in to replace the decentralized nature that cars introduce is a monumental and perhaps untenable task.