Banks explains, in one the books, how the universe somehow produces humanoids all over the place, for unknown reasons; probably it's simply an optimal shape in a certain type of gravity and atmosphere; and that this is why many of the species in the books are humanoid. It's a writer's way out, really, and a perfectly adequate one.
Banks has other weird and interesting species appearing in the periphery. But the books are ultimately not about weird aliens.
He also goes further in this explanation in another work (whose name eludes me), in so far as some races deliberately seed the galaxy with humans so that when a race such as ours breaks free of their solar system, they don't get all hegemonic because there are already humans everywhere.
