This might be a classic case of "this game just isn't for me". There's a staggering number of reviews that strongly disagree with your viewpoint:
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Personally, I think the gameplay is pretty mid. Nothing really new in the Soulslike genre. But the exploration, animation quality, and overall vast array of kinks being covered really makes it stand out, whereas many H game kind of just focus on a small handful of lewd concepts. The game is crammed full of enemy animations, game over CGs, cutscenes, and background NPCs, to a degree that I've rarely seen in other games. This makes it appeal to a wide variety of tastes. I've seen people discussing things from bestiality to mind control to bondage to futas to exhibitionism to clones, all of which cleanly fit into the context of the base game's offerings.
It also is clearly inspired by Bloodborne (and dark fantasy in general), which is arguably one of From Soft's most popular settings. It was also very visibly in the works for several years, allowing it to amass quite a following before even being released.
That all, plus the fact that it runs in unity using Spine animation makes it relatively straightforward for players to take apart and mod, adding to the longevity of the content. It's definitely not a perfect game by any means, but being totally flabbergasted that a game is popular despite you personally not finding it impressive is... actually kind of standard in the gaming community lol but coming into the thread full of dedicated fans to the game just to comment "this game is really mid lol, why are you all talking about it?" is likely going to get you some sour responses.