For the same reason we don't expose children to potentially addictive substances, we should not expose them to potentially addictive social media.

Each parent should se their own rules but sometimes folks don't do that and we end up with a lot of adults who can't take care of themselves cuz no one ever showed them how

Well to entertain the offtopic games point, yes it's a 3d game, but it also isn't. It completely depends on what you're actually doing inside Roblox. You can play extremely challenging parkour games, or literally just stand around in some lobby. To add onto that, it's often played on tablets and phones, where all controls are condensed into left and right thumb controls.

And even giving that point to you that yes, some games are good for development, it isn't at all related to the social media ban. I picked roblox specifically because of the chat function it has, and changed, to dodge the Australian age limit for social media.

An easier example would be discord, where, because of its more private nature, grooming has gone absolutely wild (youtube link). In these cases restricting their accounts to just DM's would already help a ton, but having nuanced solutions like that for every platform that serves minors is nearly impossible I think.