I think there is multiple factors here.

First, Europe wasn’t a major weapon producer, and ramping up production takes time. You have to recruit people, teach them, and buy heavy machinery, which also take time to produce.

Secondly, Europe isn’t as unified s the US. It formed around the idea of preventing future wars, and on that aspect it worked spectacularly. There was no war between EU countries since WW2. But cultural differences are still there, and its institutions require a consensus, not a majority. So it is hard to take immediate decision. Ironically, USA withdrawal is what made the consensus easier to reach, because major countries that were reticent became aware that they had to act now or fold to Putin.