It's usually low IQs or autists
Take home grown mangaka lolcow
Mono as an example.
He's afraid of his work being "stolen," which makes no sense as everything on the internet doesn't really belong to you in a traditional sense. As an artist you do want your work to be proliferated, the few who do like your work will also most likely seek out who made that work so that they can follow you.
Low IQ people can't figure the internet out, so you see it with kids posting "my oc do not steal" and autism vortexes like Japanese circles tend to be territorial about people reposting their shit.