Well, Germany was in an all-out war that was fought partially on their soil. It was very obvious to the Nazi leadership that there was a very real risk of losing that war.
That's not comparable at all to the wars the USA or Russia have fought in this century. Even in WW2 there was never any serious risk of anyone invading the USA.
The last time the USA has had a war on their territory that actually threatened the existance of the USA was, I guess, the civil war.
If it was about the existance of the USA, they'd also be spending a little more on the war effort.
(Just to make sure that nobody misunderstands me: Nazis are scum and as an Austrian I am really happy they lost the war. I was just talking about defence spending strategy.)