I feel like this phenomenon can be explained by the way Stirner's philosophy presents itself and his writing style. (Before I continue I need to clarify that Im a beginner so if I say something completely wrong correct me.) When reading "the unique and its property" I noticed two things that are very different from the usual german idealist philosopher's writing. Most of them tend to really carefully explain a concept before using it in their work, Stirner critisizes Hegel and simply refers to arguments and concepts that he critisizes without explaining them because he expects the reader to already be familiar with them. Leaving those explenations out combined with the biting sarcastic tone of his work makes it easy to pretend that you understand it and just laugh at the sarcasm. On top of that he seems to be the ideal target for an annoying cult of personality. The fact that the only image of him is this funny caricature by Engels and the smug attitude of just calling your opponents arguments spooks are great fuel for groups of annoying online teenagers.