The issue is that right now those platforms can't really exist. Even if the platforms themselves had an incentive structure to do so, credit card processors would cut them off or heavily punish them in fees and rates.
Let's say you wanted to legally sell NSFW content. Perfectly legal, nothing morally objectionable. Well, you can't host it on several server providers immediately because their TOS/AUP restrict it. Some providers may allow it with significant restrictions.
Once you find a place to host, you still need to accept money. Paypal, Amazon, Google Pay, et al are right out. You can't use Stripe last I checked. You may be able to use Authorize.net or another middleman but you'll have to post a bond and pay a much higher rate. They may still cut you off.
And that's all for perfectly legal non-morally questionable content! That's for porn which 80%+ of the population indulges in.
> Nobody's preventing them, as an outside force
Except practically yes, they are.
And the above platforms DO exist, people CAN use them, and by God there's no shortage of porn on the internet.
(ccbill for NSFW billing, btw)