Oh, I think I see what's confusing you. In the xkcd comic, the pebbles by themselves aren't a universe, it's the pebbles being interpreted by the right interpreter that are the universe. The right interpreter is simply a mechanism that (at its simplest) is caused to do action X by a pebble, and action Y by there not being a pebble, where X never equals Y.

So yes, somebody does actually have to bother doing the calculation, because the calculation is the universe (or consciousness, or whatever).