Comment by wolvoleo

Because 2^128 is too big to be reasonably filled even if you give a ip address to every grain of sand. 64 bits is good enough for network routing and 64 bits for the host to auto configure an ip address is a bonus feature. The reason why 64 bits is because it large enough for no collisions with picking a ephemeral random number or and it can fit your 48 bit mac address if you want a consistent number.

With a fixed size host identifier compared to a variable size ipv4 host identifier network renumbering becomes easier. If you separate out the host part of the ip address a network operator can change ip ranges by simply replacing the top 64 bits with prefix translation and other computers can still be routed to with the unique bottom 64 bits in the new ip network.

This is what you do if you start with a clean sheet and design a protocol where you don't need to put address scarcity as the first priority.

Yeah, the current system is really weird, with many address assigning services refusing to create smaller pools. I really hope that's fixed one day. We already got an RFC saying effectively "going back to classful ranges was stupid" https://datatracker.ietf.org/doc/html/rfc6177 (for over a decade...)

Point of fact it's giving 4 billion Internets worth of addresses to every local subnet.

You will sometimes see admins complain that IPv6 demands that you allow ICMP (at least the TOOBIG messages) through the firewall because they're worried that people on the internet will start doing pingscans of their network. This is because they do not understand what 2^64 is.