In addition to all the good points made here (designs are static, different devices, etc) it's also because WYSIWYG editors just are plain inefficient. It's faster and easier to work with text. The best tools and technology work on text (copy/paste, git, sass, etc). Most people use HTML/CSS IDEs the provide the same level of power as programming language IDEs.
The OP is also talking about editing the visual side of things, and I think if a well-done editor that also output good CSS and HTML existed, it would be embraced by many.
Of course, I'm someone who enjoys using Xcode's interface builder tools to edit GUIs, rather than laying out controls in code, because I get instant feedback with Xcode's interface builder. As others have mentioned, something similar for Web work seems like it could be very powerful.