You can thank all local train operators for this. They have been fighting a shared ticketing system tooth and nail at the European level and the weak politicians in Europe who don't push for a shared system.
What is the rationale for fighting a unified system? A unified system would make it easier to travel by train, which should in theory encourage people to do so more.
Is this a problem of the operators within each country not wanting to be unified with each other because then they'd have to compete more directly? Or is this actually the operators between countries fighting over it for some reason?
There is a legislative proposal but that will take years and operators are going to try and get around it: https://www.europarl.europa.eu/legislative-train/spotlight-J...