Of course you're right about this but a defensive bonus for castles and strongholds would only encourage turtling and make it a very boring game. The siege engines on the other hand, encourage offensive play which makes the game more interesting.
Beside that, castles still need to be organized. Just walking in didnt gave benefits. They had to repair the buildings, man the towers, put archers to the arrowslits etc.. This game kinda portrays this with defensive orders, which can only placed on areas where units already had time to do such things. If they just arrived there, they didnt had time for that (no new order tokens before start of next round, basically one month later or so).
Of course defensive orders on castles and strongholds would get more bonus than such on wood or field areas then, but they tried to integrate that more or less with some house cards that give bonusses on castles. :/