But this is not one subnet. It is a large number of IPs distributed across a bunch of providers, and handled possibly by dozens if not hundreds of routers along the way. Each of these routers won't have trouble blocking a dozen or two IPs that would be currently involved in a DDoS attack.

But this would require a service like DNSBL / RBL which email providers use. Mutually trusting big players would exchange lists of IPs currently involved in DDoS attacks, and block them way downstream in their networks, a few hops from the originating machines. They could even notify the affected customers.

But this would require a lot of work to build, and a serious amount of care to operate correctly and efficiently. ISPs don't seem to have a monetary incentive to do that.