Because, as I said:

layer 7 firewalls for the network which are going to be where most the majority of attacks are concentrated.

The NAT doesn't have to operate at layer 7 to be effective for this because

coincidentally it is doing the heavy lifting for home network security because it is dropping packets from connections originating from outside the network, barring of course, forwarded ports and DMZ hosts because the router has no idea where to route them.

The point is that the SPI firewalls are not protecting against the majority of the attacks we've seen for decades now from botnets and other arbitrary sources of attacks, except, perhaps targeted DDoSing which isn't the big problems for most home networks. They must worry about having their OS' and software exploited and owned in the background, which doesn't get much of an assist from a router's firewall.

Obviously, this is however true for the NAT since the NAT are going to drop connections originating from outside the network attempting to communicate with that software to exploit it

barring of course, forwarded ports and DMZ hosts because the router has no idea where to route them.