Short version: because it needs work whether it's a sunny day or the last embers of the burning world are cooling, and every condition in between. It needs to allow formatting of an arbitrary string, while not being able to allocate arbitrary memory when called, or take any locks, for that matter. Oh, and it needs to not slow the system to a crawl. And to be useful, its output has to end up somewhere a person can see it.

It would be a miracle if it wasn't incredibly complicated.