No, quite the opposite. Nearly every router has a very simple option to not allow packets from IPs not on the same CIDR as the interface. If ISPs turned this on

No, quite the opposite. Nearly every router has a very simple option to not allow packets from IPs not on the same CIDR as the interface. If ISPs turned this on on each edge interface, the problem would be solved. It's entirely political/economic. There is no incentive for an ISP to do so because it provides no direct benefit. It's a classic issue of externalities.