Because most of the time, the reveal immediately results in a magical resolution of decades of behavioral issues stemming from the trauma.
Which never happens in real life. A reveal is a huge step in the healing process but it doesn't mean the issue goes away. Maybe we cope better, maybe we regress maybe we relapse. But its seldom the clean outcome in the movies.
If a tiny 2 foot long cookie cutter shark can take chunks out of people, then any shark bigger than that with sharp teeth is equally if not more dangerous.