Howitzers are built to fire at an upward angle -- perhaps less accurate overall when going up against a target where both the gun platform and the enemy are moving? The bazooka, on the other hand, fired at a flat trajectory.
Usually howitzers fire, observe where the shell lands and then adapt. Here this couldn't work as ships changed positions.
In other words, you are right. Having something that you can point and shoot is easier to do. This might not be true if they could aim their howitzer low but then you get the fact that howitzer cannot be steadied (it will roll with the ship) while a human holding a bazooka can do some stabilization and fire at an exact moment the target is in crosshairs.
Howitzer works as a mortar or if they are lucky and can aim low, as a non stabilized gun.