Plus, of course, each European country has to support their own defense industry, so each one of them needs to have their own howitzer/tank/whatever and they can't agree on common approach that would actually allow for the economy of scale.
We will see whether the new realities of the war in Ukraine and Trump's approach to Europe will substantially change that.
The whole economic and societal tensions in the west make things tougher...
https://time.com/archive/6731121/how-clinton-decided-on-nato...
NATO wanted to be deliberate and slow about admitting any new members, but countries that wanted to join felt that anyone who didn't join might get attacked or face hybrid measures from Russia to prevent them from joining next. So they grouped up and 7 countries joined NATO simultaneously. NATO was never begging them to join, they wanted to join NATO.
People push this vision of NATO being some hungry bastard that can't get enough, but it's largely outside pressures pushing countries to want to join it.
Sure enough they were right. Russia invaded both Georgia and Ukraine, which wanted to join already because Russia kept interfering in their societies.