Because that argument can have no refutation, jor any clear verification. You wont find a control group that lives the same life as these kids but without a cell phone. You can't teleport these kids 50 years earlier etc.
So we're bound to hear hot takes that have no other substanciation than "it makes logical sense"
My main issue with these hot takes is that it doesn't help, as smartphone aren't going away.
In contrast, we already understand the effects of Instagram on teenagers for instance, but nothing is actually happening on this front. So this whole debate on wether "smartphones" are to blame seems like a distraction from a more actionable "why isn't Instagram more regulated"