Because even if not explicitly stated, much of those attitudes are implicitly ladened into our society and culture. A holdover from less enlightened times maybe, or just a naturally emergent result of the confluence of a great many people of a great variability of ability.
A lonely guy without a social group of peers to defuse his echo chamber of thoughts could concievably pick up on all these implicit cues and over obsess on them to the point where it seemed as though this *is* how the world operates (and how everyone else believes it to also be the case).
It's remarkable how distorted his world view was... but I'm sure he's not unique in his views, unfortunate as that may be.