Historically, field artillery was a relatively rare bird until the 16th-17th century in Europe. Light torsion-powered bolt throwers appear in roman and medieval times, yes, but even then they were rather thin on the ground. When cannon appeared in the 14th century, it made the occasional appearance in field battles, as at Crecy and Flodden and whatnot, and became increasingly more common.
Two minor points... One is the "new" Burgundian cannons in the 15th century, and Chuck5's bigass artillery train. Or trains, since he kept losing them. Didn't the 15th century Ottomans have piles and piles of cannon, too?
Also, 16th century artillery *is* relevent, since that's the period that's, uh... "inspired" the Empire army in the first place.
In any case, WFB mixes eras and at some point made some attempt to model some real world physics into the rules in the early days, which is where the cannon and mortar came from. Granted, doom divers, pigeon bombs and robo-horses have sort of thrown that all out the window... but I'm not sure the cannon and mortar rules are that far in fantasy-land.
Everything in the game gets abstracted to a certain extent. This is then tempered with tweaks to balance the rules within the game. I remember when artillery could fire indirectly. I though that was fine. They changed the rules at some point, I'm fine with that. It's a game. Instead of complaining about having to shoot at a goblin screen, I'll either find a hill, find a better target, or find a different way to deal with the situation as it's happening.
Oh, and thanks Captain Tineal, Perambulator and Warlord.
After typing that I drew out plans for a T-Shirt including that reference. I'd like to make one like this:
