(a) fixed width does not mean unusable (or should I say "fixed widths.") Not even close.
(b) real users also want no advertising. If we are talking about just giving people everything for free and everything that they want, you're right.
(c) the only sensible decision is one which people can easily understand and reduce to simple terms. With fixed widths you'd have every advertiser needing to discuss screen size, how the ad may shrink, where it goes when screens are made wider or reduced and other variables. Every additional variable adds another part to the equation and nobody wants to get short changed. By having limited but perfectly accepted widths, order and expectations are regular and expectations are maintained.
I found that everything in the world is a sacrifice of sorts.