I am not sure if you genuinely don't see what you miss or if you purposely ignore it because it helps your argument.
Nobody says "we ban it for children and then expect them to use it responsibly". Just like for cigarettes. It's never responsible to smoke, it's just that we as a society choose to let adults stink and ruin their health if they want to. But for kids, we a society believe that they are vulnerable and may smoke without realising how stupid and unhealthy it is, so we want to protect them until they reach an age where it becomes harder to justify preventing people from taking those stupid decisions.
For social media it's exactly the same: the goal is to protect the children while they are vulnerable, until they reach an age where it becomes harder to justify. Not that many (most?) adults are wasting a big part of their life swiping utterly stupid stuff on their smartphone, but banning social media for adults is a completely different discussion.
The thing is, if a kid is the only one in their class to not have a smartphone or to not have access to social media, they are in an uncomfortable situation and that puts pressure on the parents to give them access to those, so that they can conform. A big part of being a kid is to conform: being an outlier is a risk. One reason for age verification is to prevent enough kids to access social media, such that "the norm" is suddenly not to have access to social media anymore.
You can disagree with that, but at least you could acknowledge the argument instead of going around it and talking like if there was absolutely no way a sane person could imagine that banning social media for kids could ever be constructive.