Because the ability to have different sound levels on stage vs. in front of house is invaluable even in tiny venues.
It's totally feasible to, and I have done this before, have a thin nasally clacky bass sound on stage so it cuts through at low volumes, and send a bottom heavy sound through the subs.
Check with a good sound guy sometime; you'll see that the monitors are drastically different sounding than the front of house, and it's for the same reason you may want a different bass sound on stage (either volume or eq).
And I'll add to that...
How much extra cash and/or work does it really take to make that much of a difference in the quality of sound? I've heard quite a few bands that sounded together musically and their sound out front was crap (either by lack of PA, or one that couldn't handle the entire band). and it ruined what could have been an otherwise great night of music.
It takes so little extra effort to get a full, blended sound in any venue (large or small) that it makes no sense to me not having a PA handle the job it was intended for.


