I agree its jarring, but its because most platforms have strict rules forbidding anything to do with incest. If they get caught then they get kicked off the site and can earn no more money.
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Sometimes they care and sometimes they dont, Ive certainly come across posts within the last couple of months indicating that different creators have had files removed from mega download, been kicked off Patreon, been contacted by Patreon staff and asked to change game content to comply with the rules etc. So I have no doubt that its a real risk, even if every infraction isnt punished immediately.
If its your only form of income then how willing would you be to risk it?