Many regions where it would not make sense economically to connect high speed trains can be connected with electric plains. For many regions you simply don't need more then a small number flights.

Trains make sense when you need transport very large numbers of people.

Airplanes can be dynamically allocated where needed much easier.

And the simply fact is, despite massively subsidized rail infrastructure, even current planes are the cheaper option. Electric trains will require far less maintenance and will use electricity.

Legally speaking building a train line across developed region is an absolute nightmare. It takes a very long time, often isn't successful at all and the project takes so long that financing interest is a killer.

Comparatively a really quite electric plane is easy, far easier then current planes.

The far faster speed of planes is preferable to most people. Sure there are super fast trains, but those are really only viable on a incredibly small set of routes.