Why Do Americans Think That All English People Are Posh?

Why do Americans think that all English people have a posh accent? We don’t. The only people in England who have a posh accent are the people who live in London. All of my American cousins say to me that I have a posh accent, but I don’t and in American movies if there is someone who is English, they make them sound so posh like the royal family, but not everyone in England talks like that.