Why dedicated /64 to anything? We are getting by just fine on /32 for the whole world right now. Why is a sudden expansion of 2^32 required RIGHT NOW?
Stateless autoconfiguration.
See, that's the classic mistake: Everyone wants to divide the entire address space RIGHT NOW, without any clue as to how the world will evolve in years to come. Nature may abhor a vacuum, but it certainly also seems that engineers abhor unallocated address space, and will try to allocate everything even when they are lightyears off the mark
About 85% of the IPv6 address space is specifically left unused at this time. And even within the 2000::/3 which is defined for global unicast use *now* just 3/8192th is really used.