Oh, but the screenshot was you sieging a castle, so I thought you meant the computer-controlled catapults on your side as "the AI".
If AI is besieging, then, from my observation, it is often what you said - building engines to destroy walls and waiting.
They may be never attacking if the odds aren't good enough in their favor, which kind of proves that they aren't assaulting blindly.
They will forever build catapults and then disperse, so the problem is actually that AI doesn't use "put in reserve" feature.
And/or maybe whole siege engine building needs balancing/rework. Sadly, this "put in reserve" feels like a "mod" feature that has to work around limitations of an original game.