That assumes that the world is naturally described in four dimensions (the three primary spacial ones and time) - but that's just a habit of humans. It's perfectly valid to describe space-time in one dimension or, if you're a bosonic string theorist, 26. The habit of identifying three dimensions (and leaving out obvious things like spin) is just one that's indoctrinated into us by our common mode of education. 3+1D is certainly very sensible - but it isn't the only right answer by a long shot. And, again, 2D is much more socially common - going to 4D really just feels like being pointlessly obscure.