Probably because sometimes you want to check libraries into your project. For custom jars, I'd immagine you'd want to specify their paths exactly, or something like (bin|build)/*.jar
Can't you exclude all jars by default and add whatever libs you do want to check in? I don't know too much about git, but that is how I would do it in Mercurial. (In Mercurial, if you specify an explicit filename, it overrides .hgignore.)
