Non-Americans can put on an American accent much better than American people putting on a British accent?

Americans sound a bit too fake with British accents, I'm talking about well known actors who can't do one well. However, you'd be suprised so many British/Aussie actors can do an American one so well?
Update: Ahh, that was meant to be 'why do you think...' but it was too long ^^
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