I think it has to do with something like the fact that they start life as trees, and as such don't always grow in nice straight comfortable lines. As they are cut, dried, and then glued up with other non similar pieces of wood (read pieces of trees) they don't always behave like we'd want them to. Hence, warps, bows, live and dead spots, etc. Most of this can be adjusted by design, but not always all the time.
Kris