You can, but then you have to choose between having your fixed-dimension resources look absolutely terrible when scaled to very high resolutions, o rhave everyone download the super-huge resource and then scale it down for median resolutions - which (in addition to being a waste of bandwidth) can still look ugly by introducing moire effects from the disparity between actual and displayed resolution.
People tend to make the choice that looks good for the most amount of visitors. Ideally, you’re looking at individual site statistics, because different sorts of sites attract different sorts of browsers. If you’re not looking at site statistics, you can use Google’s little tool to give you a general idea.
About one in ten people use a screen width wider than 1440, and one in twenty use one narrower than 1024. Nobody cares too much about the little old ladies that are still running 800X600, so let 'em scroll. The assumption is that if you have your screen set to something like 1920X1080, you are probably browsing in a window on a big-ass screen. The bulk of sites are going to be designed to look best for the majority of people.
If laying everything out to go edge-to-edge from the highest resolution to the lowest possible were a good design choice, it would be a lot more common - but (unless your content lies within a very narrow range of purely scalable,) it is a bad choice because you are trading a benefit to a small minority for flaws that will be apparent to the majority.
