Because the web is meant for documents first, not applications.
The example who linked to is a nice demo, and I understand how the GSS is meant to be used. But for all the fancy CSS techniques that hit the HN frontpage (and that are usually frowned upon because there is no "practical" use case), there's probably one just as suitable for this particular situation.
A somewhat related example, with just a few JS lines: http://toki-woki.net/lab/fluid-corners/