What on earth gives you that idea? Why does only ARIN have this
information? Just because it happens to reside in their whois server?
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.