Because it is not German propaganda from a century ago. It is a peace plan from a century ago, that imagined a world in which the Second World War never need have happened.
Huh, how peaceful.
Tribalism (Nationalism) is the cause or justification for most wars, no? Us against Them. Whether it's Nations or Religions or Races. I think tribalism is so deeply rooted in most of our cultures, that ideas that challenge it are hard to process for most people. I think there's a lot of value in challenging assumptions. This specific proposal has a whole host of problems with it, but there is still merit in the idea of dealing with the cultural forces of nationalism.
No. It may be a banner under which a specific group is united, but it is never a cause.
The urge to take over resources that someone else controls is.
But it doesn't work without being able to convince other people that they'll benefit from joining that cause, based on being members of your group/nation. Those wars are justified typically by generating a shared sense of fear and/or hate for the other side. If people's ties or shared identity to that group were weaker, it would be harder for people to buy into that propaganda.
I find this interesting, because without some unifying forces I don't think our modern society would be able to function well. I'm just not sure Geography and national identity/culture are the best way to unify people. Can't say that I have any better alternatives though, other than thinking that if our personal identities were less tied to our national identity, the world might be a little friendlier place.
For creating a us/them group, any shared/diving trait can be used. It can be tribal, religion, or it can be class (bourgeoisie vs workers), or ahem intersectional.
It is exactly violations of those borders which got us wars in Europe. Sometimes for the sake of colonization, sometimes for the sake of unification, sometimes for the sake of ill-conceived peace, and almost always a bad idea in the end.
I'm not even that much optimistic about the future of the EU, to be honest.