Internet routers have burdens if they run full routing tables. They may not have the time to filter incoming BGP traffic due to the burden they already have. And providers don't want to pay to upgrade to bigger routers.

This level of the Internet works on the KISS principle. It works because it is at a simple level, and the rules don't take many cycles to implement, unlike what it would take to code Asimov's three laws of robotics.

What folks are missing is that Pakistan is to blame. It was their responsibility to block outbound BGP announcement of that custom black-hole route. What we need is economic sanctions of some reasonable measure on the ISP to properly encourage good citizenship of the Internet.