Why do they serve it at Chinese restaurants?
When your family has emigrated (or been forced to emigrate, or bribed to emigrate, or tricked into emigrating under false pretenses, all of which happened to Chinese people recruited for the U.S. labor market in the late 1800's, as opposed to those who emigrated to join the California Gold Rush), your labor options in the new country are often very limited. Food service is one major area of opportunity, which is why ethnic restaurants are a mainstay of any given immigrant section of any given town. Adapting one's home cuisine for local palates allows one to get some money back out of the surrounding community, as adventuresome eaters 'discover' this new cuisine.
The Chinese, in particular, had some incentives to go into food service - the Union Pacific Railway, which recruited heavily in the Chinese immigrant community, paid their Chinese workers the same as their white counterparts, but provided food for the white laborers. The Chinese had to provide their own...
And a number of dishes were invented during this era, as certain traditional Chinese ingredients were in short supply. Chow Mein was developed as a way to dump leftovers. Fortune Cookies were developed in San Francisco (and by a Japanese restaurant, to boot). Crab Rangoons have already been mentioned, and a lot of other traditional dishes were adapted for local conditions and tastes. There are still Chinese restaurants in Denver, for example, that have a published menu for the general public, and a menu in Chinese for Chinese customers, offering stuff that's much less popular with Americans (100 year eggs, for example).
In short - it's marketing. You sell what sells, even if it's not actually "Chinese". Same reason you find Buffalo Wings in Seattle, or fish and chips at any bar that pretends to be Irish/English/Scottish... it's what people expect to find.
As for my personal favorite, it's gotta be Har Gau (shrimp dumplings).