The only thing the merchant cares about is CAC. If a customer walks in on their own, and they can acquire their business for less than the affiliate fee, then why not?
If some affiliate is actually doing the work of obtaining the customer to earn the fee, but then the customer has an economic incentive to strip the affiliate attribution to get the fee for themselves, then the affiliate isn’t getting paid. Maybe they’ll recommend another product instead, which doesn’t incentivize users to break the affiliate link.