Because NEC 210.12 requires all devices to be protected. Which means if you have a switch or splice before a plug the only way to protect those is with an AFCI

Because NEC 210.12 requires all devices to be protected. Which means if you have a switch or splice before a plug the only way to protect those is with an AFCI breaker. The only exception is a continuous run from the breaker to an outlet in metal conduit or MC cable. Given how much is romex this effectively forces AFCI branch breakers.