Good question. One way to think about this is: why don't you or I use a GUI to build a front end. I tried to use one for Bootstrap, and it was hard to get the everything to line up right. I wasn't sure how things would act when the screen changed, or how to change that. Then when I went into the CSS it was laid out in an unfamiliar fashion. These problems seem very solvable, but I don't know why they persist.
(this also applies to anyone else who thinks that GUI frontend editing is the way forward - send me an email and I'll let you know when I have something to show you! I won't send you anything else, scout's honour).