Because according to HN there's no way to force social media companies to not serve children without destroying privacy. That leaves parents who aren't changing their behavior because nothing has incited them to change. Legal liabily is an incentive that changes behavior.
Maybe not everyone agrees that social media causes literal cancer. I mean it’s not great, but it’s not the worst thing on earth, either. It’s clearly become a moral panic at this point.
If we define cancer as "that which replicates without regard to its host" then social media has actually caused a huge number of cancers.