Because parasocial relationships with ewhores isn't healthy, particularly at a stage in their life when they should be forming real relationships with their peers.
Scrolling through attractive women (generally the thirst-traps are women) doesn't imply forming a parasocial relationship. I agree that parasocial relationships are bad, but this is independent of them being thirst-traps. Internet thirst-traps are just the modern equivalent of sneaking a look at a playboy mag or a lingerie catalogue. Nothing inherently damaging about it. The scale of modern social media can make otherwise innocuous stimuli damaging, but this is also independent of it being content of sexy women.
You are the one claiming there's a problem, and you are the one (presumably) demanding legal and other action to deal with that "problem". That means that any burden of proof is 1000 percent on you.
... and before you haul out crap from BYU or whatever, be aware that some of us have actually read that work and know how poor it is.
Parasocial relationships are a different topic than pornography.
Are you saying that the intersection is uniquely bad? In either case limits to content made in an effort to minimize parasocial relationships cut across very different lines than if the goal is minimizing access to porn.