@Monceber
I think a lot of them forget that he isn't actually the Demon King, but one of his generals, as a lot of comedic stories with similar set ups focus more on "The Hero and the Demon Lord", with any generals being side characters who are usually introduced after the Demon Lord, as opposed to this one, where the Demon King hasn't even appeared (and is not even mentioned all that often)
That and he just looks like a Demon Lord (though most comedic stories I've read that feature nice demon lords tend to have them be more human looking).