Because it excludes most forms of interactivity.

After looking it up I see it is commonly used to describe something that adds extra features to a browser, like flash or silverlight.

Previously I took it to mean "rich in interactivity". That's what I meant when I originally wrote that, a static site does not mean you can't have interactive content. IMHO the best way to write webapps is to have a static client that consumes an API provided by a standalone backend.