It is true that it's simple, but complexity and depth are not the same things at all.
I flatly disagree that Settlers is a shallow game. It is true that it's simple, but complexity and depth are not the same things at all. Despite this great simplicity, there are several different paths to winning the game, with a variety of overlapping strategies. Play benefits from careful planning, but requires constant adaptation. And again, the social element adds as many layers as you and your fellow players bring to the game.