Just 'cuz, that's why
My husband asks me last night, "What's the difference between body cream and body lotion?"
So I try my best to explain that body cream (and we're talking about Bath and Body Works products, here) is thicker, and in most cases, stronger smelling, and that the lotion is smoother and not as strong scented.

Then came the million dollar question, to which I had no answer: "Why do you need both?"