Cos that's what they're called.

They're somewhat rarely called ducks as well. They're basically just herbs, off-cuts and offal. Pretty decent way of eating stuff that would get chucked away otherwise. Kinda like haggis or black pudding in that regard.

I assume (and I couldn't find much info sadly) that the name comes from the same way a bundle of sticks is named. It's a bunch of stuff bound together.