Because there is hardly any public funding for anime and manga in Japan, animators are at the mercy of the free market, and given the demographics and sexual mores of Japanese society, producing erotic wish-fulfillment is not just a, but the safe bet for any profit-driven studio. Consumers of such products tend to ignore the story, worldbuilding, characters, creative directing and storyboarding, general aesthetics, or the quality of the animation; therefore the studios which supply them tend not to be interested in such things either, which makes their products much less labour-intensive and hence cheaper and faster to churn out. I won’t name any names, but it is not coincidental that such studios also have the worst work environments in the industry. This is why there are like 5 good shows every season, and the whole rest either goes to the dogs together with the chequebooks of its former studio (which is a shame because often they butcher and waste good source materials), or is forgettable dogshit produced by studios which are good at accounting, marketing, then serving unsuspecting first-timers the most recent mutation of their usual formulaic crap, and little else.