I suspect that the Ethernet switch is not part of the SoC, so it's a separate component that is already price-segmented. A consumer wireless router with 8+ Ethernet ports would probably not have significantly better margins than the existing models, but might need another round of FCC certification, and might also cannibalize sales of enterprise routers.
Also, I think most consumers who have a need for more than 4 computers to connect to their router via Ethernet probably don't want their entire network to have such an expensive single point of failure. I've had Ethernet ports on wireless routers die before. I like my switch separate from my router/AP separate from my modem.