The answer is that the system doesn’t work if a 3%-fee card isn’t held by a low-risk, high-spend rich person. Indeed if that weren’t the case, merchants would reject the tiered fee structure.
(This is also the answer as to why in the absence of regulation, exchange fees aren’t higher than they already are.)
(This is also the answer as to why in the absence of regulation, exchange fees aren’t higher than they already are.)