We can establish social norms, but companies have a tendency to ignore those norms if it makes them money and it isn't illegal (maybe not all or even most companies, but if it's profitable, some company will do it and expand into that niche). So it makes sense to make it illegal for those companies to provide services to children, and then establish a social norm that parents won't create an account for their children/bypass the checks that companies need to do. Just like with alcohol: it is illegal for stores to sell it to minors, and they must check ID; we don't just let them shrug and say a 14 year old looked 21, and at least in the US, that would be a criminal offense. It's then socially unacceptable (and maybe also illegal) for a parent to buy a ton of alcohol so their kid can host a rager for all of their friends.

Drawing out the alcohol analogy further, you can actually buy alcohol on Amazon, subject to an ID check. I'm not sure why no one bats an eye at this, but somehow e.g. porn or other adult-only services are different.

It's long been an established, reasonable stance that it is both the parent's responsibility and decision to allow or deny certain things, and it's also illegal for businesses to completely undermine the parent's ability to act as that gatekeeper for their kids.