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/
That train has left the station long ago.
Which is, ironically, why something much better than CSS is necessary. Right now you simply can't have rich presentation and a presentation independent document because CSS just doesn't actually give you that. Need to re-arrange your document's presentation with CSS? You'll have to move your html around to make that happen. Need a grid layout? Throw a bunch of stupid classes all over your html for your ridiculous CSS framework to latch onto. To rearrange them? Switch them around.
A richer presentation layout will help the web be for documents first, not hinder it. And more so than trying to pretend that it's still 1994 and no one's ever tried to do anything remotely complex on the web yet.
Compare a nicely designed magazine or a book to a typical website. Which one is more readable? The reason why so much of the web is not nice to read is because complex layouts in CSS are hard.
The web, which doesn't have well-defined pages or even page sizes, precise control over typography, or designers who agonize over how the actual text appears in the context of the layout, lacks all of those. Typography is slowly getting better, but text layout algorithms are still relatively primitive.
Books with diagrams and side-notes tend to have more complex layouts, sure - and that's most educational books. Maybe there these typography issues are given more attention, but it's pretty clear that the typical paperback has demonstrated they aren't that important to readers (which isn't a judgement, it's just the way it is), so I wouldn't be surprised if almost no books worry about these issues anymore, beyond perhaps using software which addresses them insofar as they can be automatically.
Where have you been for the past decade? The web is the world's largest app platform, and it's not going away any time soon.
iOS developers have the power of using constraints for their layouts. Why not give the same to web development?
As for fluidity, this is something GSS can provide very easily with the conditionals. Imagine being able to do media query-style rules, but not only for the screen size but also for sizes of other elements. "If the image is bigger than X, change the layout so-and-so".
A little example: open the toolbar layout demo (http://gridstylesheets.org/demos/apple/) in a browser window, and resize that around. See how the button that prefers to be in the center, but even more strongly doesn't want to overlap with the other buttons behaves.