There's a few reasons.
It fairly recently has started relaxing the censorship. There hasn't really been enough time for anyone to put their games on the store. Also, a lot of games have content that still can't be put on there. Steam has said that anything is allowed unless it's illegal, but they also have taken upon themselves to decide what that means. From what I can tell, that includes games with a protagonist who looks like they're young, whether it be stated they're over 18 or not, and even incest, which is a very popular theme right now. Because of this, many game still can't be on steam. It's still fairly difficult to get an adult game onto steam. Some get accepted, others don't, and there's not a clear explanation why.
Plus, a lot of developers release their game for free during development. In order to release on steam, they'd have to completely develop the game without releasing any builds to the public, which means no funding through patreon. Which means it can really only be developed in their free time. Making people pay for the final version of a game on steam after getting supported through patreon would also just be an enormous dick move. People could very easily just find the free versions through sites like this, and not buy it too. Not to mention that a lot of developers use the builds to find bugs.
Some developers also use assets that they didn't acquire legally. Stuff like music, art, and DAZ assets. In order to release on steam, they'd have to acquire the rights to all of those assets. Anyone using something like Unity would also have to pay in order to release it.
Also, anyone who made a parody game could never release on steam. It would just be asking for the game to get removed and to be possibly sued. Companies generally don't care if something is released for free, but start making money off of it and they will take notice.
Lastly, the adult games community is used to getting things for free. Most people are just going to come to a site like this to get the game for free anyway, so it's just a better idea to just use patreon instead. People who want to support you will, those that don't, won't. And, to me, that's a way better way of going about adult games.
tl;dr making a game to release on Steam is just a bad idea. If the approval process were to be more transparent, and people would be willing to make full games in their free time/if it were possible to get investors to fund an adult game, then maybe it would be a good idea. But as it is now, it's too risky and you likely aren't going to make much money anyway.