Technically pretty much every router does have built in porn blocking, the problem is is that it's across the board.

With these routers it's almost always All or nothing.

It's also slightly complicated to set up in the first place, and the grand majority of people will not spend more than 10 seconds setting up any technology unless they absolutely have to.

Most people will not go through the process of finding out what the IP address of their router is, attempting to log into it with the default passwords available on the internet, navigating through the HTML 1.0 1993 interface to find the section that allows them to enable parental controls and then enabling them.