90% of threads like this that I see seem to be from the perspective of people who have never tried making something like this. You don't need to be a professional chef to tell if something tastes good, or if it's overpriced or whatever, but only an idiot would argue with a professional chef about the challenges you face in a professional kitchen just because they've eaten at a few different restaurants. It reads as "Why can't you just caremelise the onions faster?"
It's not a perfect analogy since you don't pay for a subscription to eat at a restaurant, but you get the idea. Businesses start based on initial investement. If you don't have an initial investement, such is the case with most games like this, how can it be sustained without subscriptions? Sure, devs can work on the game for free in just their spare time, and most do I'd say, but it's exactly that. It's work. Not many people are willing to work for thousands of hours, for free, on the off chance that what they're working on might make some money in a few years once it's done. And that's assuming it doesn't get shut down or denied by the sales or payment platform being used for being a porn game.
This isn't Japan, the west doesn't have the same legitimised smut industry that they do. Nobody is paying salaries to devs to make porn games unless those salaries come from subscription money. (Or microtransactions like with Nutaku games)
This kind of thing just isn't a good or safe investment in the west. Take away all the games currently in development on this site, and I imagine the next thread would be "Why isn't anyone releasing any games?"