Jim raises an extremely good point.

Why not start ARIN off by giving it a /8 to work with and see how well
it will actually work?

Lets face it, there is only ONE /0, and giving that to ARIN might not
be a good idea if it doesn't work right. By giving them a /8, they
will have to compete with other ways of assigning IP addresses, and
demonstrate that they actually can work.

Just like a would-be ISP has to start with a /24 and work up...