Instead of why not? Ask why? It's a marketing gimmick.
I don't know how much of a curve, how many your curves, where your curves are at, or if you have any curves, to make a curved belt. And if you wear your pants around your knees...what kind of curve does that take?
Personally, I don't want my pants forced to droop or rise on the sides, the back, or in the front. I like my pants to be even all the way around. I used to make gun belts and wear one I made for myself. I like the professional look of a straight belt being worn.
If you want to get into curved gun belts, have the customer send you how many, where, degree of curvature, up/down of the curves they want, and a picture and see if "a straight belt will do" is the answer. I know my wife and I don't have "the same curves and a "one size fits all" does not when applied to curved belts.