Why do scientist so strongly discredit sea monster sightings?

Well i had a thought, why do sciesntist so strongly discredit these sightings? I mean we have explored i think less then 4% of the ocean, and i mean alligators, crocodiles, snakes, sharks, turtles, fishes and frogs have been around since the dinosaurs. So whats do say a plesiosaur hasnt been evolving with us this whole time and we just didnt know it?