as someone else pointed out, it's bc published works are basically pre-vetted

that doesn't mean they're good, blah blah fifty shades of whatever etc etc, but it does mean multiple people looked it over and said "a decent number of readers won't immediately write this off as schlocky," and then, on top of that, a prospective rper of a canon has to read it and find a character interesting to someone other than the author

none of that is true of ocs by default: they have no editors, and absolutely no one has to find them interesting apart from their creator -- so it's quite possible that the author of xyz manga is using characters or story beats to work through stuff, but other people have gone over it and found it interesting anyway