>> Since this is HN, it’s 2021 and DDoS’es are still a thing: why are they still a thing? Is there some fundamental “anonymity” to the Internet that makes it impossible to structurally prevent DDoS attacks?

IMHO that is exactly what the problem is. Weather its malware, phishing scams, extortion schemes, the fundamental problem is knowing where stuff comes from.

I propose an IPv6 subnet that routes by using a portion of the IP address as lat/long coordinates as a starting point because routing tables are litterally a form of indirection.