Because a WYSIWYG Editor (in design view, obviously) is like a "servant" cooking up what you want, but it may not always be the way that you want it... therefore you (the main chef) have to do it yourself to get it the way you want it.

Many WYSIWYG Editors cant handle complex css layouts, and tend to break. I find handcoding much faster (and having 125+ wpm helps), you don't have to waste your time changing what the Editor chose to put in for you, you put it all in there yourself, and you know exactly what elements you used, where you used them, what styles you applied to them, etc... and this definitely helps in troubleshooting.

That's why I prefer handcoding.