Because unless a particularly amazing song is playing during the credits sequence, I'm officially done watching the movie when the words start scrolling. Putting an extra scene at the end comes off as self-indulgent, anyway, and that just makes me hate these movies even more.

I sat through a decent portion of the credits even before "after credits" scenes became popular. And then, of course, once you know (from hearing everyone talk about the ones after, for example, Iron Man 1 and Hulk) that they're going to be there, you of course sit and watch. Anything else would be dumb.
(Oh, by the way - Brave has an after-credits scene. Damn funny one, too. If you see that movie, try to remember not to storm out of the theatre in a rage before it happens.)