I think it's because some visual mediums are better at representing certain forms of information than others, and visual programming tools attempt to represent a form of information (programming logic) using a visual medium (images) that isn't optimal for that form of information. Analogies can be made for things like calendars (which are the preferred visual medium for visualizing schedules), sheet music (which is the preferred visual medium for visualizing music - though guitar tabs present an interesting alternative to think about), and movie scripts (for which text is the preferred medium).

In the same way that a movie script is best represented by text, my belief is that text is superior to images for representing programming logic, and images are superior to text for representing UI elements. I think most people at least feel the same about representing UI elements, which is why visual programming tools are appealing.

This leads to the conclusion that the best tools for building UIs would represent programming logic in text, and represent UI elements using images. In my experience, this is true; tools like Unity and Flash are, in my opinion, better for UI development than HTML/CSS (text). This also leads to the conclusion that unless someone invents a better way of representing programming logic than using text - like the equivalent of inventing a calendar to display a schedule, if we had never invented a calendar before - visual-only programming is inherently limited.

Tools that keep UI in visual-land and programming logic in text-land - and don't try to mix the two - work great.