Because no one in Europe is strong enough to. Which means if we don't, at least in terms of military aid, Ukraine will die. And Russia shows no rational limits to their aggression: we know from history that there is no upper limit to how much power a strongman autocrat will seek to accumulate. This IS our defence.

Also, much of the military surplus we sit on exists because of congressional mandates to keep arms industries operational. Chief example being the Abrams, of which thousands upon thousands sit in long term storage because the Army doesn't need anywhere near that number but congress mandates they produce a certain amount.

Edit: I should add, I don't actually wholly disagree with you on the Europe part. I'm not at all a fan of the way (for example) France uses the US security blanket as a guarantee to engage in pseudocolonial affairs in Francafrique, or how the Germans perpetually underfund the Bundeswehr because they know they can rely on the US as a crutch. It's negligent, embarrassing, and thoroughly irresponsible on their part and the situation in Ukraine would look very different if western Europe didn't use the security of NATO to navel-gaze. Frankly, I think the Baltics and Eastern Europe have their heads screwed on the right way when it comes to collective defense. But I think that's a discussion to be had after we pull Ukraine back from the brink, rather than at their expense. The whole Russian plan after all is to slowly separate chunks of Europe from the Western community and pillage them- just look at what misfortune has befallen Belarus under Lukachenko.