I'm starting to suspect that the engine Larian is using just can't support the ability to use a player action to move a spell effect, which is why all summons are their own separate controllable entity. The engine also doesn't support true flight, which is why you have Harpies jumping everywhere, Flaming Sphere rolls on the ground, and so on and so forth. It would not be too much of a stretch to believe that Spiritual Weapon would work in exactly the same way. (Coincidentally, there is no example of true flight in D:OS2 either, there was a spell that allowed you to sprout wings in that game, but all it did was to give you the ability to spend an extra point of AP to jump huge distances for about 3 turns.)
You’re absolutely right and it’s only occurring to me now that Levitate, Fly, Telekinesis, Bigby’s Hand and many other spells are going to be ruined in this engine. This saddens me greatly.