I mean, an EU army is hardly the main reason to leave the EU. But this was always about sovereignty and democracy. Why the hell would you expect anyone who values sovereignty to be on board with an EU army? It's a further erosion of nation state sovereignty, it weakens a nation while strengthening the bloc. It makes it even more difficult to leave the EU in the future. I'm not sure why you're surprised that it's a relevant matter when we talk about EU membership.