Summary once again, because I think you're going in a roundabout way.

You assume:

a) Companies will by necessity pay people doing ostensibly the same thing different amounts of money

b) People do not necessarily wish for their salary to be made public

c) Making a salary range requires the lower and upper bound to be equidistant from the expected starting pay

d) Making too narrow a salary range would therefore reveal pay, in contradiction to b).

Therefore, it is a necessity of employees' desired privacy and employers' ability to advertise the lowest, er, "most competitive" wage possible for the company to have a wide salary range.

EDIT to add: I think your argument still falls a little flat, because salary ranges should still be accurate. "We would consider hiring someone with slightly less resume experience for the bottom range, we would hire a rockstar demigod at the highend, and just what we're looking for somewhere around the middle".