The live food we can get, alive or frozen, is somewhat limited in choice. Frogs in the wild don't live on a diet of bloodworm alone, something you often see quoted as the only food frogs need. In the wild they would have a wide variety of live food which they don't get in captivity which is why it needs to be supplemented with specialist food. Fish food (flake, pellet etc) is formulated to suit the needs of fish; frog food is formulated to supply what the frog gets in the wild that bloodworm alone can't provide.
I'm not saying don't feed bloodworm but they do need something else as well. Fish fed exclusively on bloodworm would not be healthy yet so many sites say bloodworm is all frogs should eat.
Frogs also enjoy daphnia and brine shrimps.