AFAIK, their war lay primarily in the Pacific, and beyond supporting the Brits and Russians materially, I’m not really sure why the US would want to involve themselves physically in the European theatre. I do feel fear of Germans beating them to the bomb might have something to do with it, but that’s just conjecture.
Nazi Germany was the logical endpoint of US ideology. Hitler explicitly stated he was inspired by the US's birth via genocide and expansionist colonialism, and the whole fascist project in general is a last-ditch effort to save capitalism from the threat of a socialist revolution. The US is evil but it's not stupid, it knew enough to step in before it got to the point of explicit fascism on their own part.