Probably for the same reason that the curtains separating the cabins must be completely open during take-off and landing. Of course, that could be solved technically (wall that slides downward to open the compartment). But I don't think there are train compartments with reclining seats -> less comfort and more effort for the crews during service. There is also a potential safety issue: air-rage in a closed compartment might be stopped too late.

I've never seen a train ride that costs 9000 Euro for a 10 hour trip... If it did cost even half that much I'd almost expect my whole carriage for myself...

There is no doubt for me that lie-flat seats are a fabulous idea on long-hauls.

HTB.