Maybe it's because they're not trying to make money off of it? I don't think they run ads on NSFW subreddits.
I wonder if there's a market for something like how strip clubs operate -- where the artist pays OnlyFans to stream there, and then the artist collects their income some other way (e.g. Patreon, or just direct cash equivalents). That could in theory keep OnlyFans good with their payment processors, while still keeping this revenue stream at least somewhat open.