I think some people expect a big battle scene from fantasy films. There isn't one in VDT, SC, and MN. Though it would seem that Hollywood would want to squeeze it in there.
While HHB and LB each have a battle, neither are "saving the world" plot.
In HHB, saving Narnia and Archenland from the invading the Calormenes is more of the "B-plot". Though it could get turned into the "A-plot."
In LB, it is about the end of Narnia, so what would be the point of making it about saving the world? It could turned into one, though. They would probably want Rishda build up his army of Calormenes to invade Narnia and have Tirian and his Narnians fight back.