They are joining because NATO is seen as being part of the EU. This is what Russia wanted to avoid when it proposed its European Security Treaty in Munich in 2010 (and re-proposed every year since).
Europe needs its own defense force and its own security framework. NATO was designed, as Lord Imsay said 60 years ago, ‘Keep the Russians out, the Americans in, and the Germans down.’
Obviously that is not longer what is needed.
NATO is just a weapon the US uses to "keep the Russians out", but modern Europe - especially Germany, built its economy on cheap and plentiful Russian gas, an a friendly relationship / partnership with Russia.
Driving a wedge between Germany and Russia is just the US's way to "keep the Germans down".
https://www.unz.com/mwhitney/the-crisis-in-ukraine-is-not-about-ukraine-its-about-germany/
In fact, Germany and France had been pushing for a more European defense strategy "independent of the US".
