The main difference is that a gambler will keep throwing money (and encouraged to do so via dark patterns to keep the margins), whereas post-nut clarity is the main barrier to maintain the margins in porn.
What I don’t understand is the selective enforcement – why doesn’t this happen more often? Who is pressuring the payment networks?
In general merchants don't deal with client cards directly but through payment processors. The chargeback process is usually manual (meaning it requires non-negligible resources) plus it impacts the fees the card cartel collects (if the chargeback is accepted).
High risk payment processors generally agree to pay card cartels for the fee lost if chargebacks are accepted, and sometimes even perform refunds to stay within a threshold of chargebacks. This, plus extra fraud monitoring/protection makes their service more expensive.
You don't need a processor in Monaco, CCBill or PaymentCloud are US based and offer that. Now, they shave a significant chunk of the transaction in flat+percentage fees, it can be anywhere from 4 to 15% depending on your profile.