There is good reason they don't do that: trains derail once in a while, your plan means the following train will hit that train and so the accident is worse.
Part of the answer to that is better track maintenance. However that isn't a perfect answer and so we need larger gaps.
Note that cars on the freeway are normally much closer together than is safe as well. If cars maintained a safe following distance we would need 5 times as many lanes. (you know the massive freeways in Huston that urbanists like to show as bad: that is about the correct size of freeway for Des Moines, Huston needs many more if they want to be a car oriented city)
As soon as one train has to slow down, you're going to get the mother of all cascading effects - trains are slow to brake, but their acceleration is quite a bit slower than that, and you'll be limited to, at the very best, the acceleration of the slowest train in front of you.
Saying trains should be closer than braking distance is like saying with devs shouldn’t support https. It’s something no expert in the field wound agree with.
Plus as soon as you need to throw a set of points between successive trains, you're back to absolute braking distance, because a set of point in transit effectively acts as a stationary obstacle.
