2 reasons - One is that most of the time, it's a waste. I don't make a whole lot of things that require sizing to be perfect. The few sweaters I make are better served starting the sleeve as the swatch. After all, by the time you've finished the swatch, you could be 1/4 of the way up a sleeve instead if you're right, and I've typically been right when I pick my yarn and needles. I also tend to knit a lot of socks, which don't require gauge, and other things like bags and blankets and wraps, which don't really require gauge either.
The other reason is that swatches lie, or I make the swatch perfectly but for some other reason it doesn't actually help with the making of the item. Take this swatch for example:
It's for my mother in law's cardigan. She wants a sexy, curve-skimming cardigan that can be worn with or without layers underneath. Something with, say 1/2" of ease and a ribbed mock-turtleneck type neck. Originally, when I was planning to finish it before coming to Japan, I swatched with three different needle sizes in the round, as I was going to knit it in the round and steek the front open to toss in the zipper. Now that I don't have the person who it's for here to try it on, I'm going to be knitting it in pieces and seaming it, so that if it doesn't fit for some reason, I can take it apart and fix it. You cannot fix a steeked sweater very easily, if at all.
It's so sad. I even carried the yarn 'round the back to make it all knit the same way I would be knitting it, using much yarn in the process. I blocked it too! Knit and blocked and done the right way, only to be wrong. Oh well. I must make another. I am not using a pattern on this one, else I wouldn't bother, but I shall do it right this time, so help me Bob!