For a more united defence industry to make sense, you need a united foreign policy. Some things have happend, but with decision requiring all members to agree that becomes basicly impossible. Right now far right parties are in power in Italy, Poland, Hungary, Sweden and Finland and I propably forgot some. They are not going to agree to those kinds of decisions.

Without such agreements, you end up without a plan for a military, which dictates the set up of weapons. Take tanks for example Leclerc is a much lighter tank then Leopard2 due to being designed to be transported to Africa to fight there. Obviously you can transport Leopard2s as well, but they are heavier, which does matter a lot less, as they were designed by a divided Germany, who only really thought about fighting a war against a Soviet invasion. So wihtout a common foreign policy there can be not common military police and no common defence industry.

We really need treaty reform and if that means a small core EU agreeing on setting something like that up. Idk France, Germany, Spain and a number of other smaller countries. Depends on what is possible, but it needs to be done.