It is because the IPv6 rollout has not been consistent. Some assign /64 per machine, some assign /64 per data center. Some even go the other way and do a /56 per machine. We've had to build up a list of overrides to do some ranges by /64 and others by /128 because of how they allocate addresses. This creates extra burden on server operators and it's not surprising that some just choose not to deal with it.
What can you do to get a new IPv6 network that is easier than getting a new IPv4?
Stuff like bouncing a modem, getting a new VPS, making a VPN connection I would expect to be pretty similar. And getting a block officially allocated to you is a lot of work.
Why are we pretending that you are checking logs and adding firewall rules manually. Anything worth ddosing is going to have automatic systems that take care of this. If not put an ai agent on it.