That's a completely unhelpful, overly simplistic straw man argument.
We restrict certain activities and places in the real world from certain people all the time. For example, not allowing people under 18 or 21 (depending on your country) into casinos. What we have now is essentially unrestricted access to pretty much anything and a fair assessment is that there is societal harm from that. We're creating gambling addicts (which is arguably the most harmful form of addiction), allowing predators to interact with children,, manipulating children through advertising and algorithms, flaming harmful behaviors like eating disorders, allowing mass cyberbullying and so on.
So saying "we should allow unfettered access to the internet" or even "it's the parents' responsibility" is naive, dismissive and has failed. The only question from here is what to d we do about it. You can say "nothing" but that's a losing argument.
I personally believe that the easiest thign to attack is advertising to minors. This will take away the financial incentive for these platforms to create addictive behaivors in minors. And most of these tech platforms have already built the infrastructure to do this. You don't allow advertisers to target an audience based on (actual or inferred) ages under 18. You extend that to proxies for age, like an interest in Minecraft. And you make advertising to children illegal.
Arguably, I'd go further and restrict certain features for minors, such as comments on Youtube and an algorithmic feed.
At the moment nobody is solving anything because it's simply a fight to move liability to someone else. Meta wants hardware vendors to be responsible because, guess what?, they have no hardware platform. Apple and Google likely want app to have to deal with it for the complete opposite reason.
I believe we should shift that liability to advertising.