If R=V/I defines the resistance, why do you call it a law?
Well, it becomes a useful law if you can say
that R has a definite value. And sometimes it
does. A metal wire is Ohmic in a wide range
of practical situations.
Of course all bets are off in case of
-- high frequency (skin depth and all that)
-- funny geometries (e.g. inductors)
-- high current density (heating)
-- tiny samples (mean free path effects, junction mess)
-- nonmetals
-- superconductors
-- etc.