Why Is It?

I've read a lot of scripts by beginning screenwriters. (The reason why is not important, but let's just say that it would not be inappropriate to feel pity for me.) And now I'm working on a movie by (I think he is) a first-time writer/director, and the movie has the failings one might expect from a person who, perhaps - I'm just guessing - has never written a screenplay before. And so I have one question: why is it that, almost without fail, the main character in a beginner's script is the least interesting person in the whole movie?

I'm not kidding. I'm not even exaggerating to make a point. It's ridiculously common. First-time screenwriter? First draft of a screenplay? Dull lead character. Deadly dull. Things may happen to him or her, but to no effect. The main character is almost invariably passive, practically inert, lackadaisical to the point of pathology. And, as a result, so is the script. Or, in the instant case, the movie.

I wonder if anyone else in a position to read way too much first-draft, amateur fiction has noticed this phenomenon.