Nearly all of the subway lines were built many decades ago when construction costs were much cheaper. It’s basically impossible for San Francisco to ever build such a comprehensive system at this point. Consider that they just spent $2 billion on a train station that doesn’t have any train tracks serving it and was only open for six weeks before it was closed because it was found to be structurally unsound. If they do ever build the train tracks (estimated to take ten years and cost $6 billion), they didn’t build enough platforms for the number of trains they want to run.