I can’t give an answer as to why America is so Neville Chamberlain-like, but things have gone far enough where there are no quick or painless solutions to be had.
I think part of the answer comes from why Neville Chamberlain himself acted as he did: war fatigue. Chamberlain recoiled from the horrors of the Great War of 1914-1918, and did not want to see such a thing repeated.
America today has that same war fatigue. After Afghanistan and Iraq, few Americans want another open conflict. There is little support for one, necessary or not.
Of course, part of the answer also comes from the fools at the top who are negotiating for America. But that's another story.