Trains are fine but also very expensive. And rail networks are already very congested. And also very outdated in large parts of Europe. People like the idea of trains but not the practice of high ticket prices, disruptive delays, multi day journeys to cover the distance you can fly in 2 hours, etc.

The main issue with aviation is that fuel is burned, which is bad for the environment. However a lot of short haul flight can transition to battery electric pretty soon. Probably before any significant amount of new rail gets built. That removes the major environmental concerns. And lots of small planes can fly point to point so you remove the dependence on big central hubs as well.