A lot of designers right now (like ronaldj in another comment) think that it's not practical or unnecessary. They think that the design will come out too rigid or it's too subjective.
But the thing is, as someone who just does a few pages once in a while, I don't care about this - I just want to be able to throw up a button on a page, choose it's place/size/color, and then be able to just say "add rounded corners" with a checkbox, instead of having to research on the web the various ways to do that, and finding out why certain solutions will work, but not in IE6, and why JS solutions will work and have their own problems, etc.
Such a tool will open up a whole new set of people into web design, which is a huge net plus.
By the way, matt1 from HN has built a tool that's working on doing this: www.leandesigns.com. It's not perfect yet, but it's already pretty great and saves me lots of time. Check it out.
We had all that in tools like Powerbuilder, back in the 1990s. AFAIK you still have that if you build your site using Flash (though I've never used Flash to build anything, so I don't know).
The world decided that browser-based apps were better, warts and all. In a way it's a shame that plugin vendors didn't take seriously the concepts of security, performance, and openness, the web could have ended up a very different place.
Likewise, more developers would probably be happy to use WYSIWYG layout tools if they did not generate hideously formatted, immensely verbose and complex markup that is nearly impossible to hand-tweak when needed, or if the tools themselves were not so complex that mastering them was just as much work as mastering HTML/CSS.
I would be interested in a tool that lets me visually build a basic page layout, something like a wireframe, and generates clean markup and CSS that I then easily enhance/fine tune by hand. I'll check out this Lean Designs tool that's been mentioned a few time here...