In my opinion Steam has the worst hentai games and on top of that you have to pay for them as well
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lol. But they do have them and they do get paid. Itch should jump on that and steal those customers away. As you pointed out, there are good hentai games on itch and many bad ones on steam (not that itch would not have those bad games as well ;-) And as I can testify, itch is not very well known.
If you look at some adult games, many of them here are trying to get to steam or are already there. It is a big market and publishing on itch only gives a trickle of exposure in comparison. Of course they might still publish a "public" version or pay what you want version and only sell the privilege of having the higher version number of the game a month early.