Which makes sense - you have buyers and sellers who insist on anonymity, services that leave no trace once rendered, buyers and sellers lying to family and friends about what they're doing, etc.
There's often no "normal" amount of consumption, for example, some sellers receive million dollar tips.
Money laundering is a massive problem, and it enables some really terrible things.
I suspect the fact that American banks are so anti-porn comes from the fact that the American financial sector has such strong anti-money laundering regs (as opposed to, say, the American real estate sector, or the UK financial sector).
One of the reasons OF is doing well is because they insist on following know your customer laws. Not many porn platforms could function that way.