What you have to understand is that pretty much every shark movie is predicated on the fish at hand behaving in ways that sharks never do. They don’t hunt people as a general rule, and they certainly don’t fixate on individual humans to the point of ignoring other, more fish-friendly prey (even the seminal shark flick Jaws is guilty of this, though The Shallows is the silliest and most enjoyable example).
“Realism” in a shark flick is a spurious notion; rather, movie sharks are avatars of our primal fear of being further down the food chain than we’d like. Plausibility is a moot point when your brainstem is dealing with the visceral terror of being eaten alive.
Still, Great White takes a stab at explaining why the titular apex predator is noshing on its ensemble of attractive young folks. Former marine biologist Charlie (Aaron Jakubenko) wonders if it’s something to do with shrinking fish populations (mature Great Whites largely prey on seals, son) or rising water temperatures, or some such. We don’t get an answer, and fair enough: people are, after all, being eaten, which is a more immediate concern.