>>790
Because
1. There's a tendency to pattern the dialogue and characters after western 3D porn instead of imagining normal people having sex.
2. Source material for fanart tends to be more on the full body cartoony side instead of the relatively realistically proportioned bodies with cartoony heads you see in eastern 2D porn.
2. b) Corollary, poor anatomy/shading; lack of imaginative composition, camera angles to maximize erotic appeal.
3. No large-scale enthusiast creator conventions like Comiket for buying and selling drawn porn, so no crucible for young artists to refine their craft in the competition of the open market. Without a place to earn money for erotic works there's no incentive to self-improve to earn a bigger piece of the pie. Western artists are free to wallow in their own crapulence thanks to their only outlet for publishing being unpaid online hugbox cultures such as deviantart. The limited paid work that exists being single illustration comissions from the lowest of the underserved fetish groups who are so happy to have their fantasy fulfilled that they're not picky about quality.