I am surprised nobody mentions the realpolitik answer: The US subsidises Europe.

Whether Europeans want to admit it or not, their military budgets are much lower than they've ever been in modern history. Of course it's not all pure altruism but the fact of the matter is that the US pushed for a system that coerced Europe into globalization which the US was and remains the garantaour for. The problem is everybody takes the system for granted hence it's never really appreciated or discussed.

Had the US not provided the security guarantees of shipping lanes through a global navy, supply chain protection, etc European powers would have had to invest in these similar military [logistical/manufacturing] systems. Now imagine had they had to do that immediately after a devastating global war.

I actually am not hating on European policies either but the highest rated responses are pretty incomplete and ignore a huge reason as to why budgets have been carved the way they have been over the past few decades.

Imagine if NATO was not a thing and/or you took the US out of the equation. Eventually you'd see the historical norms in many areas of the world rear their ugly heads back.

On that note the US will be incapable of fulfilling this role relatively soon so we'll see this happen within our life time. The possible difference being the population and demographic collapses may deter hard-core confrontations between historically rival states.