The bottom line is that gameplay is just too hard to develop for some random coomer dev who ends up on this site. Bad gameplay is worse than no gameplay. But first, we have to determine what gameplay is in the context of a porn game.
We could say making choices in a VN could be considered "gameplay" for a VN. I actually do think this is true to an extent (especially for complex code), but for the purposes of this discussion, let's say that gameplay is something beyond a standard VN.
We could say that the purpose of the gameplay is "fun." What is fun is different for everyone, but for the most part, there should at least be a purpose to the gameplay—something that it achieves within the context of the game and story. It's not really fun when you feel like you are doing a mindless task that doesn't achieve anything meaningful.
Some people feel that getting some pussy at the end of a mindless grind is rewarding and fun in and of itself. That's kind of a bad take, but whatever. We'll give those "repeat scene 10 times to get a slight variation" type of stuff a pass for now. What I can't give a pass to are the linear sandboxes that has only one place you can go to trigger the next event in a linear chain. Doubly so if all the locations are pretty much empty except the place you need to go at that moment. At minimum, the idea of "sandbox" gameplay in these VNs should be to offer some amount of freedom to the player. Who does he pursue? What order does he do things? Can you get suboptimal results if you didn't do things perfectly? Can you get different outcomes based on how you did things? If you don't even do this, what exactly are we walking around the house for?
Getting back to the "fun" part, I do think the best gameplay are things that are inherently enjoyable to do on its own. "Hey, I came here for porn, but the gameplay elements are actually enjoyable enough on its own." That should be the ultimate goal. Using pussy as an incentive to drive players to engage in the gameplay is fine, as long as the gameplay itself is interesting.
Of course, the best gameplay eroges are from big studios from Japan, but there are indie devs that do quite an admirable job. Void's Calling, Tales of Terrara, High-Rise Climb, Vae Victus (I think, I only played the first release), and a handful of others in Renpy. The Last Sovereign, Renryuu, Roundscape and I'm sure many others that I've not played yet in western RPGM list. Evening Starter, Pronant Symphony, Leviathan, Lulu Farea and many others that have legit gameplay from smaller Japanese circles.
I think conceptualizing and implementing a gameplay system that makes sense is probably more skill based than any other aspect of development. If it's done poorly, it can only detract from the experience. For the most part, I'd say that the hate for sandbox and general aversion to grind/gameplay is just the natural outcome from so many devs botching the execution.