Doing a cooktop and wall oven is more expensive than doing a range. Also, placing a wall oven under a cooktop makes it be lower to the ground than the one in the range. It's harder to get stuff in and out of. Your counter costs more because you're now paying for the whole section plus a cutout rather than two pieces on either side. Traditionally this has only been done because somone wanted a gas cooktop with an electric oven. With modern dual fuel appliances available, most of that reason has gone by the wayside.

I'd strongly suggest exploring an induction range rather than gas. It's as responsive as gas, but with less heat lost into the home.