Ah yes but then the content needs to be flexible enough to scale as well - which can be a massive issue in itself. It is a difficult concept for many people to grasp (designer or client). I've seen the look of horror some people get when they watch their website shift as you move the broswer window... after they ask why the paragraph of text isn't the same depth of the image next to it.
The web isn't print, but people feel more comfortable when it attempts to imitate it.