Most non-native English speakers around the world will be taught British rather than American English, so this 'accent' would be what you could expect from them in real life if they were to speak English.
I would also partly chalk it up to the ubiquity of American TV and movies. The midwestern accent is the norm for spoken English on TV, and most of those are set in America with American characters. If you want to say someone is from somewhere else but they’re all speaking a non-English language, speaking in a British accent sounds “foreign enough”.
