URI's

Could somebody please explain to me why IP addresses are not URI's?  It sort
of seems like web browsers and web servers think that they are.

I realize that there are some IP addresses used locally for NAT and so on,
so maybe I'm asking whether the subset of IP addresses that are defined
globally are URI's.  Or perhaps in one-to-one correspondance to URI's by the
trivial adding of http:// <http://>  at the beginning.