When I asked my older son this question his immediate reply was "Well, because multiplication is commutative". Which is as good as saying "because that's what it is, why won't you stop asking obvious questions?". He's been aware for a while of how convenient this property of multiplications is: I would ask him "how much is a thousand times three?", and instead of adding three, plus three, plus three a thousand times he would say "the same as three times a thousand, three thousand". The article.