I was wondering the same thing. I was about to write pastas. I wrote pastas in a later exercise and was marked correct, however, it did not seem correct to say They are eating my pastas so I went with pasta.
"Pastas" is not a very common word. It's like fishes. "Fishes" is occasionally used to refer to multiple types of fish, but "fish" is used for referring to a group of fish or a single fish. I would think it is a similar case with pasta.