Wherever you find a community of young people writing, you will find Mary-Sues. It’s just one of those things that some young people (and a few older people, too) seem to accidentally fall into! I think it’s because they use their writing to live out their fantasies, so they make a perfect version of themselves!
But goodness can it be frustrating for readers. Have you come across some awful mary-sue characters? What do you hate about them? Complain away!