I should explain a bit about the type of work I do, so this will make more sense.
I'm a technical writer. I spend most of my editing time dealing with text, not tags -- but the text is full of tables, lists, headings, anchors, and other stuff that gives the document structure in a browser, but makes it look like word soup in the HTML. This type of file is really impossible to read in the sense that you'd read a book.
Thus when I revise an HTML document with a typical editor, my work cycle looks like this:
Locate something I need to change in the browser window.
Choose something near-by that's distinctive enough to search for.
Search the HTML file for it.
Figure out whether the instance I found is the right one, and search again if not.
Locate the near-by thing that I actually want to edit.
Typically I spend most of my time on the first five steps, leaving not much for the sixth.
I want a tool that will do the scut work and let me concentrate on the stuff that I'm paid to know how to do. Specifically, I want to be able to click something in the browser window and have the insert point go to the corresponding place in the HTML file. If I can actually edit the text in the browser window, that's nice, but it's not really important. I'll switch back to the code window anyway if I need to do anything more than the most elementary tag editing.
If the browser window does a poor job of duplicating what I'd see in a real browser, that's too bad, but it's also not important. If the browser window is accurate enough to let me navigate by using my eyes instead of a search command, that's all I need. I don't have to know exactly what the text will look like in a browser. The style sheet takes care of that.
My work is different from that of a web designer, who mostly works with tags, and it's also different from the work of a CMS user, who cares so little about tags that markdown language will do. I don't think there's a good term for the tool I need, but I'll call it a WYSIWYG HTML editor for back of anything better.
It may be that no one has created one of these because code editors on one hand and CMS editors on the other are "the tools of the trade these days." But that's just a way of saying, "It's like this because this is what it's like." I don't believe my needs are so exotic that there's no demand for tools to serve them. (Nor, I notice, does Adobe.)
I also don't believe that a WYSIWG HTML editor would be too difficult for the open source community to create. Open source software includes some of the most sophisticated products available today: OpenOffice, for example, and Linux. If the open source community decides to create a WYSIWYG HTML editor, it will.