Why can't countries print another country's currency?
Why assuming that it isn't done in the first place? Turns out, it was actually done by Nazis in WWII

https://en.wikipedia.org/wiki/Operation_Bernhard

Also, suppose that somebody actually made dollars that are indistinguishable from genuine, Fed-printed dollars. How would you find out in the first place? How sure are you that nobody is doing it? :)