The USA have been at war all of this century, and while militarily winning, because they have militarily won, there have also been servicemen killed, maimed, and forgotten by the government who sent them to fight when they return. That is why common Americans don't want more engagements now. Trump would be fine with a war against PR China and both him and Biden against Iran. Many Republicans want to militarize the southern border. But neither would support a war, any war, in Europe. And each of these potential actions would have clear political costs for the guy deciding it even if other constituents would approve it. That is why upon explicit Ukrainian demands for USA military engagement the response has been in the negative. The only actual pro Ukraine response I've seen from America was when federal Senator Graham proposed in his sites and in the press the assassination of Putin. Even if they did so, which I doubt they will if they have not in a month (and which US law forbids; not that they have followed it...), it would be a very targeted response, not a full blown war, and American casualties would be zero to minimal.
In any case Europe also put itself in a weak position by depending on Russian energy sources through abandoning nuclear and not buying enough from the Middle East, Africa, other former Soviet countries and Venezuela. That is another reason for American disinterest: the USA have their own domestic sources plus Canada, Mexico and Venezuela. That Putin would see this was inevitable.
Not that similar energy related issues aren't happening in other places of course.