CIDR and the resulting address space fragmentation was the problem that IPv6 was originally meant to solve; allowing splitting into random-sized subnets makes routing more complex and worse. And if you made the routable part of an IPv6 address longer than 64 bits then that makes the routing much more compute-intensive. 64 bits in the local part of the address is sort of overkill, but a 128 bit address isn't really any harder to use than a 96 bit address, and it means things like, well, being able to automatically assign the local part based on the MAC address, which only works if the local part of the address is more than 48 bits.