banker/steward is a world of difference from general/marshal. I think a lot of lords and realms would kill to be able to have the banker take care of food for them. (Yes, I know there are notable exceptions.) But I don't think you'd find too many realms where they would love to have the general be marshal of all the armies. The difference being that quite a few players love the military aspect of the game. A significantly lower number actually *enjoy* the food game. Most lords are forced to deal with it, and would rather not.
Besides, the general already *can* take control of all the armies. There is nothing in the game rules that prevents the general from issuing orders twice daily to the entire realm.
