In the first days of Hollywood films, the actors who played foreigners didn't actually have to speak the character's language. Instead they would just speak gibberish with a French or German accent & the audience would be fooled. No one spoke those langauges anyway.
Today, because this global village is so very small, those langauges are much more accessible than they were then. Many people in the US can speak two or more languages these days, so Hollywood is much more precise in its depiction of foreigners.
Still, Chinese is a language that hangs on the periphery. Other than Chinese people living in the US, it's rare to find someone who speaks Chinese as a foreign language. Most people don't really even know what Chinese sounds like beyond the stereotypical sing-song "ding dong do". If someone mimicks the sound of Chinese, they might also thrown in an "ah-so" or two.
The reason we do this is because foreign languages aren't "normal". They're not what we hear every day, so they sound strange. But to a large number of people in the world, English is the unintelligible foreign language. It's English that sounds strange.
Just what do non-English speakers hear when they listen to English? Here's a great video of an Italian pop group singing a song in gibberish meant to sound like American English. I kinda like the song. It's got a catchy beat.