Except its not actually obvious. The US achieved most when they worked mostly by themselves. And the recent explosion in US activity, specially SpaceX is also highly separated from Europe.

There is a very credible argument to be made that excessive cooperation leads to incredibly heavy overly political programs that are completely static and can't adjust because of international agreements.

That doesn't mean working together isn't good but they way they have been has been pretty questionably in many ways (along with a lot of space policy).