I dont think all social media users are children, or even the majority. But nearly all of them certainly seem to act like children online.
I am not quite sure why this happens, but if I had to guess I would say social media lends itself to very quickly becoming an echo chamber, so people only hear what they want to hear and only get compliments or likes or whatever. Before social media, people were forced to learn how to be civil and respectful to everyone, especially people you didn’t agree with. Nowadays, you just mute/block/hide them.
Why learn how to be respectful and hear people rightly tell you you’re wrong when you can just make yourself feel like a god by silencing everyone?