Obviously, being water logged and exposed to more wate rmakes it rot quicker, along with causing more rusting and maitence issues to metal equipment. (not just of the metal fixtures on engines, but weapons and armour)
Catapult, Tebuchet, Siege towers, Rams and Ladders I dont recall when the Onerger stopped being used, it may be under the peroid in question. As far as i recall, siege engines were used until replaced by Cannon, the catapult was replaced by the trebuchet, the trebuchet replaced by cannon. Not accounting for defensive works either. (of which soemthing that you can launch a dozen rocks at a attacking formation from behind your wall is useful)
And again as far as i recall for how a Catapult would be used, it would be assembled somewhere, then transported in its componet pieces with the army should they need it, same with most engines. Given there are metal parts to this, the resources may not be present at the location or you may not have time or the faciltieis to craft the wood to the specfications needed nor forge and attach the metal.
As much as you can argue for on site construction of a trebuchet, i dont think mobile craftshops had the equipment needed to do such a thing at the time. Like mobile forges couldnt make a cannon. They were more for re attaching the heads of pole arms to shafts, making shafts etc. That and if its made for a campaign, it would be pretty precisely crafted, not just a ad hoc thing. (if we move this to a trebuchet, you would need rope as well, not something easily made and would need special lines made for it)
Siege normally denotes any action against a fortification of any time. And a siege can contain a assualt or several assualts on a forification. You wouldnt call walking into a town a siege, but you would call axing a hole into a palisade a siege.
i think this is pretty far off topic at this point honestly.