>>1850187Becuase they're designed to be, duh. Well, that doesn't really answer the question of why.
Ok, serious answer, it's like the way elementals embody a certain classical element; demon girls embody abstract concepts like sex, and sex appeal. And what we think of someone affects how attractive they are.
Now there's a certain language built into these images. The physiology, the strange skin tones, they all suggest that this is something other than human, and whose drives and motivations might also very well be different. In this case, what's being communicated is that this body is built /for/ fucking. It's not a life support system for reproductive parts and genetic material with all the needs and responsibilities that entails. As such, there's an implied availability as a sex partner that doesn't exist, or doesn't come across the same way with images of regular girls.
Now you may find their exotic features interesting in their own right (I certainly do), but going back to my original point, how what we think of someone affects their attractiveness, these features are a coded transmission that communicates a readiness and willingness to fuck for its own sake. In some ways demon girls are the flip side of conventional pornography. Conventional pornography is all about power over women's bodies (just go with me on this, I'm not an SJW) with the man often being the strongest, the best looking, the most dominant, the most successful, and by virtue of that is able achieve unrestricted access to females.
Demon girls are the reverse of this, because they sell the fantasy of 'sex without having to prove yourself every god damn minute'. Which in a sense isn't just a fantasy of sex, it's a fantasy of acceptance. And acceptance is hot. This isn't something I'm pulling out of my ass here, there's a lot of science to suggest that those who like and accept us the way we are (and want to fuck us anyway) are automatically a few points out of ten more attractive.
