Who decides what is huge and/or stupid? Also, the mods would have to enforce that, and they already have more than enough to do. And inevitably there would be posters who complain and argue with the mods about rejected sigs that the posters claim are really JUST on the okay side the line.
I think that’s why it ended up being once per thread. Someone had a habit of doing gigantic sig lines or something, and it got so annoying they changed the rules. At least, that’s how I seem to remember it.