It has been asked before. Here are the main reasons there is no artillery in the game.
Assumptions (perhaps not entirely true
* Catapults and trebuchets are used against walls.
* Cat. and treb. are built at the siege site.
* Cat. and treb. are not used in regular field battles.
* Ballistae and scorpios are mainly used from walls.
* All civilized cultures have made walls and engineered means to tear them down.
Thus:
Siege equipment is made at the siege. There are no graphical representations of siege equipment, but it's there in the abstract form of the defense reduction of the besieged castle. Many strong troop work quicker when cutting woods, making rams, building catapults etc. The sappers and engineers of Ulm are remarkably good at doing stuff that is destructive to walls. Mines are placed better, stones hurled with more presicion.
There are also scorpios/ballistae in the game. Abysia, Ermor and Pythium have them in their towers instead of regular bows. Once more they are not graphically represented (apart from the slightly bigger bolt).
Still I find catapults fun, so if anyone makes a catapult mod I would probably both try and enjoy it.