Why do Americans say "tuna fish"?

I mean, it's not like there is a tuna vegetable or animal that it can be confused with.

Answer

I agree with you that it does seem redundant. However, this is common with other kinds of fish as well. Many people say "codfish" instead of "cod". Here is a recipe for "trout fish" croquets.

This convention has important meaning to a huge number of fish names: catfish, lionfish, swordfish, sunfish, cowfish, etc.

Also, it provides extra clarification for someone who wouldn't know what a "tuna" or a "cod" is otherwise. Anyone learning English as a second language will probably learn the meaning of "fish" early on, but may not know the more specific names.