That is because true hentai is mostly stolen screenshots from Japanese video games !!
It is not licenced, so nobody posessing it can sell it ...
If you can find "genuine" hentai content for sale, the person selling it to you is breaking the law.
There are a handful of artists drawing "Hentai-stle" drawings ... but most of these people are very unreliable, and their quality is nothing compared to a japanese screenshot.
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The Black Sheep of the Boneprone Family