Why is cheese never used in Americanized Chinese food?
It's simple; Americanized Chinese food rarely contains cheese because Chinese food rarely contains cheese.

As many as 90% of Chinese people are, to some degree, lactose intolerant. Dairy is simply not a large part of Chinese food culture. Dairy is growing as a business in China. However, since dairy makes most Chinese sick, I imagine the dairy industry will be primarily an industry of exportation. 

EDIT As I promised in comments, I did look for citations for that 90% figure. The figure is ubiquitous. It may in fact have more to do with societal evolution than anything innate in the genetics of the Chinese people, but the fact remains. Most Chinese people react poorly to dairy. 

Neato Chart