Why can’t people grasp the concept that America and Russia are capable of getting along with each other, EVEN during the Cold War?

I can’t speak for everyone, but generally a lot people
assume that political relations define personal relationships, when that hasn’t
been the case in the series [x]. 

It’s a weird concept to grasp if you’re just coming into the
fandom and haven’t read the manga, so I wouldn’t be so quick to fault them.