It’s not that it can’t have quality; it’s that the nature of hardcore fetish products actively discourages quality. Hardcore fetish products have a built-in base of appeal: people to whom that fetish is appealing. The more hardcore the fetish, the more built in that audience is; the more automatic sales become; the more reliable its sales figures get.
Look at what you have to do to make a good quality story. In the case of a beastgirl fetish game, the game needs to have ideally at least half a dozen beastgirls, all different types of beast. Then you need a setting for these beastgirls to inhabit. Then you have to make sure that each winnable girl has a personality that is believably connected to the animal’s traits in folklore. And even that’s not enough; you also have to give them a personality outside of that; their own personality. And then after all that, you have to come up with some plot reason for all these beastgirls to be around where they’re winnable by the MC.
Note how the reviews of (say) Shuffle suffered because of its story. It wasn’t about the setting; it wasn’t even really about the girls themselves. It was the way the story was built that was totally ludicrous; it was their reasons for being “in love” with someone most of them would probably barely even remember.
But look at this in terms of economics: A gamemaker can spend a lot of money hiring a quality writer to pen an amazing story for my hardcore fetish game that does all of the above. But that will take even a good writer a considerable amount of time to do. So that’s a lot of time and therefore money sunk into making this game.
Or … they can make a game where an evil magician has taken over the beast kingdom, captured their kings and queens, and the princesses escape to earth. In order to get back, they need … Say, vast quantities of human semen. Personalities? Well, there’s the tsundere, the yandere, the twincest, the osanajimi, etc. and you can probably line those stock characters up with the various animal types you’re including. There, that’s at least half the game right there, and if you add another couple stock points (the evil emperor’s son trying to kill you, the evil emperor’s assassin who falls in love with your pants, there are more) that can get up to like ninety percent.
Both the awesome story game, and the “wrote the plot on the john” game, require about the same music, art, VA, etc (or close enough that you can make do). But the awesome story game will cost more and take longer to develop. And how many extra sales is it really going to make you? You’re in a hardcore fetish niche; those are relatively uncrowded. So not only do you have less competition, you have a more fanatical fanbase.
This is why stuff like Berserk is so rare. Manga that has the violence and sex content of Berserk aren’t rare; but publishers and mangakas know most people buy it for the tits-n-blood. They don’t CARE about a good story; so the writers of most such manga don’t really bother. Thus you get stinkers like Apocalypse Zero.