Just the simple act of having a UPS driver pull up to your house costs money (lets say $7). Wether he lugs out a 1 pound or 50 pound box, he still had to pull over, go to your door, etc. That is why it isn't by the pound (lets say like buying bulk trail mix at the store).
I use USPS fixed rate for "heavy" light stuff that is also cheap. They have a fixed rate box that comes out to $9 no matter how heavy.
For other stuff, I usually use UPS because the nearest Kinkos is far and the nearest UPS store is close to it isn't worth it for me to save a buck or two.
Juan