I consider this to be more of a minor technical issue. ARIN can certain
provide an authoritative directory for the unallocated portions of its
own allocations. And to answer your why question; because only ARIN has
an authoritative and up-to-date view of exactly which addresses are and
are not allocated.
What on earth gives you that idea? Why does only ARIN have this
information? Just because it happens to reside in their whois server?