Lol, we've wondered the same thing! DH says it's because they can get it.
I wonder if it's because they're not as in demand anymore, at least not here. At one time, it seemed that larger more expensive houses (read more space) had the ovens separate. Then the professional style ranges became popular and I very seldom see a new home with separate ovens and cooktop. Kitchens seem to be getting smaller and that's one way to save on space plus I think there's a consumer demand for those neat looking ranges. At the lower end, they can mount the microwave over the range (I hate that) and really save space. Therefore, it's a supply and demand thing, they don't sell as many cooktops, mostly replacements. Just a guess on my part, did you ask the appliance salesman? He might actually know, lol. I love the cooktop and ovens separate myself, there are some really awesome cooktops out there, such as the Dacor mentioned, but yes, they are quite expensive