Metal improvisation doesn't work because no one ever knows what the fuck is going on. If you're seeing a fast death metal band that you've never heard before, you might find a few riffs you like, but it's too hard to digest all at once. I saw Nile before their new one came out, and it was okay hearing their new songs, but I would have thought they were way more awesome had I heard them ahead of time. Metal is all about being precise and calculated a lot of the time. The listening experience requires much more time and focus (generalization). Jamming just doesn't have a place in this. When you're jamming, the form begets the music, but in metal the music begets the form.