Don't read too much into the "sea"; there's no rule that every word in the language has to stick precisely to its etymological roots.
Seafood just means edible aquatic life, i.e. fish and shellfish in general. It's a food word, not a biology word, and fish on your plate looks pretty much the same whether it's freshwater or saltwater fish, so generally it all gets lumped into one category.
That usage might be peculiar to the US as most dictionaries like the Cambridge dictionary and Collins Dictionary list the broader definition of seafood as American English and have the narrower definition as the primary definition
Fish comes from an Old English word that meant any aquatic animal so most people would probably have just used fish without needing another word to denote all food from aquatic sources in general