Granted, I'm still a relative n00b when it comes to fantasy fiction, so I'm probably generalizing. I suppose the reason you don't see them around in fiction is because writers are probably afraid of pushing the tech level to that point because where there are cannons, guns are sure to follow. I don't think that has to be. It would be simple just to BS an excuse for firearms not to exist in a fictional world, especially since the early ones were just godawful - literally just metal tubes mounted on wood.
|Who wouldn't want to use this bad boy?|
I think writers are missing a great opportunity by not including artillery into their worlds. Imagine writing a great battle - something beyond epic - and the enemy charges across the field with their cavalry and infantry. The other side doesn't move an inch. The enemy keeps coming and coming when suddenly, BOOM! cannon balls start tearing through their lines, stopping their entire force in its tracks as the barrage keeps going. When the bombardment finally ends, the enemy army is devastated and completely collapses after the other guys stage their own charge.
Of course, artillery would have uses outside of just fiction. In wargaming, one of the factions in Warhammer Fantasy, The Empire, employs cannons. The biggest problem facing other fantasy and medieval-based wargames would be the lack of rules and how easily they could be created and implemented. Hmm. For RPGs, things could get very exciting with the addition of cannons. Imagine players being faced with the task of infiltrating a castle or fortress in order to disable the mighty guns protecting it ahead of an opposing army arriving to besiege it. On the flipside, you could task your players with defending such a place by using those same cannons.
Artillery is definitely a possibility that folks should take a second look at.