Why can't the Germans send troops into Ukraine? Or the Brits (for all their grandstanding and bluster)?
Why must it be the US?
If "everyone" thinks Putin isn't going to stop with Ukraine, then it would seem Europe has an existential crisis to deal with - not the US, thousands of miles away...
Two reasons:
1. Because whichever country does go in, it gives everyone else in NATO an excuse to do nothing in response to Russia subsequently going to full war with whoever it was that went in. Strategic vs tactical response in local political sphere, short-term tactical benefit of using such an excuse comes at long term strategic cost.
2. Right now, each individual European power is in a state of "Oh no, Russia's probably 3-5 years away from attacking us if Ukraine falls, and it will take us 5-8 years to be ready to defend ourselves". The UK, for example, has a 136k active and 32k reserve personnel in total, while Ukraine forces have suffered ~400k wounded and ~50-100k fatalities.
Both points become "we can only intervene directly as a joint act from multiple nations".
That said, if the USA were to leave NATO, I wouldn't be overly surprised if rest of NATO may issue a joint declaration that Ukraine is now under their combined protection, nor would I be overly surprised by an alternative outcome such as NATO being functionally replaced with Article 42 of the Treaty on European Union: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex%3A...
And also, much of Europe is, in fact, sending aid (and arms) to Ukraine. All the other stuff is about troops on the ground.
Even a non-nuclear direct conventional war would get very messy very quickly. NATO would have to seize Kaliningrad and Belarus, start interning Russian nationals, rush forces to hold the Baltic states, and so on. It's not complicated to say "this will be very expensive and deadly, so we're not going to do it unless absolutely forced to".
European states are now re-arming (the irony of fate is that it's now US policy for Germany and Japan to re-arm), but that is going to take a while.