No one said it, but when the limit is high like this, people tend to do it.
And that's really...you know, silly. Nobody said you need to have a sig, but a lot of people like to. Myself, I like to match my sig with my avatar (it's changed temporarily right now, don't mind it). A lot of other people like to show their interests in their sigs, or quote something awesome/funny.
I haven't seen any sig that can fill pretty much an entire page, anyway. If there was such a huge sig, surely the member guilty of it would have been notified.