Probably because more often than not the actor isn’t a native speaker so it makes more sense to just have the lines in English. The most egregious example of this is characters speaking Spanish in American movies, they often use direct translations or don’t pronounce correctly if the actor isn’t a native speaker.

Anyway, this is what I enjoy a lot about movies like Inglorious Basterds. There are two prominent scenes about the characters’ cover being blown because their non-native accent was recognized.