The problem with WYSIWYG is that it solves the wrong problem. Sure, what you see is what you are going to see, but that's irrelevant. What you care about is being able to change what you see. And that is tied up with all sorts of invisible context that you can't see.

Good interfaces tend to be based around the principle of least surprise. But in a typical WYSIWYG editor it is commonplace to be in a situation where you don't know what will be produced when you press a key or press enter. The purpose of tools is to bring complex problems under control. To make a bridge between the underlying problem space and a set of controls that are intuitive for a user. WYSIWYG does a poor job of doing that.