There are indeed firearms and cannons in Golarion, but the way they're presented in the current (and now out of print) Campaign Setting Hardcover is undergoing some significant revision. Basically, the arrival of guns and gunpowder and firearms on Golarion is changing from being a 4,000 some year fact of life to something that's much more recent. Firearms exist, but they're generally more difficult and more dangerous to make than magic items, and the secrets to their creation are mostly held by the gunsmiths of the duchy of Alkenstar—and they've had that knowledge for only about a hundred years or so. This puts Golarion, perhaps, on the cusp of entering a new age where firearms could become much more widespread... or perhaps at the cusp of an age where firearms wither out and fade into obscurity as they're overshadowed by magic. By presenting firearms in this manner, we basically enable GMs with the ability to take either direction in their own games, since we know that not everyone wants firearms to be commonplace in their games.