The policy is predicated on protecting children for their mental health and development when they are at a very vulnerable age. Not all kids have responsible and capable parents. Lots of kids live in abusive circumstances, with absent/negligent parents and some kids are forced by circumstances to effectively be the the care givers/providers in their household as their parents guardians may be incapable. The world is really fucking sad sometimes.
When you go into a pub or supermarket and ask for beer or smokes they don't give them to anyone who doesn't have a child lock on them. They ask for proof of age. You can defeat that in various ways but they too are illegal and create risks for those involved. It isn't perfect but it works well enough to reduce harms.
You want something available only to adults, then the convention is you provide proof you are an adult. That is a privacy nightmare if poorly implemented but then so is the entire digital realm right now.
I think we are missing the big opportunity as a society. The social media platforms are making shitloads of money through predatory manipulation of user habits because they get shitloads from advertising. Just ban the fucking advertising. Most of the bad shit goes away overnight for kids and adults because without the advertising the incentives to keep people trapped in a dopamine loop is mostly gone. The big platforms either learn how to produce viable paid services or people move to community run alternatives like this one.