Five reasons explain why Europe is not investing in defense at adequate levels, and all these factors would have to change if the major European states were to rearm. And that scenario is unlikely to occur.
One, the Cold War is over and there is no existential threat on the horizon remotely resembling the former Soviet Union and its Warsaw Pact that would prompt European military readiness comparable to the prior postwar period.
Two, the United States, and its contributions to NATO, subsidize the protection of Europe; America supplements the logistics and supply needs of the French and British militaries.
Three, the EU in general, and most European nations in particular, insist on redistributive socialist systems that reflect public sentiment that a dollar invested in national defense is a dollar robbed from social programs.
Four, Britain and France are nuclear and assume that fact eliminates the possibility of anything approaching World War I and II in Europe. European nuclear arms provide an adequate level of strategic deterrence that makes it unnecessary to deploy large conventional forces in Europe, or to fear an economically dominant Germany.
Five, the soft-power ideology of the EU believes war is aberrant, and can be prevented through diplomacy, sanctions, embargoes, and the UN without resort to fossilized concepts like deterrence and military readiness. Large militaries in this view only encourage military adventurism abroad and thus destabilize the global order.
None of these considerations are likely to change much, and so we should expect Europe to remain largely militarily unimpressive.