Flexible routes = unpredictable.

Flexible capacity = attempts at over optimization = not trusting that capacity will be there = unpredictable.

Shared infrastructure with roads and following road laws = all the inconvenience of traffic jams.

Flexible routes may be nice on a multi-year timetable but still runs into the jam issues above.

So if we are going to build dedicated lanes with special signal crossings and high capacity to encourage a large population to use it, let's make it as efficient as possible - a rail track is much more efficient than tires, and electricity is better than directly burning fuel, and delivering electricity through rail is better than battery.