From a practical matter, most fish in the ocean feed on other marine life. herbivorous and omnivorous fish do exist but they are a bit more rare, and it is more common for smaller/juvenile fish. Also even among those that are primarily herbivorous, they usually go after eggs or very small fish which makes them omnivorous.

The most common fish that we eat are those that other fish also eat, like herring, and there aren't a good comparable land based animal in our current diet. It would be a bit like if a primary diet for humans would be wild rodents.

One reason could be that fish like herring schools, but wild rodents don't. It is easier to hunt large quantity of animals if they are located in the same location and bunched up.