They need to respond dynamically to window resizing, have rules about which directions text boxes expand in when they overflow with text, how the page operates when a whole section is missing, etc.
Any tool that appropriately allows for all this complexity to be specified, has already become as complex as CSS itself, thus defeating the purpose.
And in my personal front-end experience, 95% of my time is spent on these harder details. Throwing together the basic HTML/CSS skeleton of a page is trivial, and so fast I don't really need a tool to speed it up, especially when that tool is always bound to get something wrong.