Unlike online casinos and bitcoin casinos where everything is anonymous on social networks it is easy enough to determine that the account belongs to an underage child.
Is it, though? Unless you enforce a KYC policy (similar to that of a casino) you can't really know. And in that case, there's no real point to doing it. It would make gambling more accessible to underage consumers, which is
not desirable.
We already have people hooked on different kinds gambling nowadays with some mobile games and loot boxes everywhere now.
Given how people can easily lie about their identity (taking pictures from the internet is as easy as writing an email) it seems a little unethical to allow gambling to be a part of such a medium. Imagine going on Twitter and having a little slot machine in the bottom-right.