2001:: is a 16 bit prefix in which every single customer could get a /48, i.e. you could allocate 65k subnets of 18.45 quint addresses on your network and give one of those to everyone with an ipv4 address and not run out, and that's still only 1/65536th of the total address space.

128 bits is a lot.

(the /64 as the physical subnet allows using hardware MAC addresses and RNG temporary/private addresses instead of needing DHCP/etc).