Online businesses, especially smaller ones, use pre-packaged shopping software, and that likely includes a space for promo/coupon codes, even if the merchant doesn't ever offer them.
(just like most bloggers today used canned packages, typically implementing AI to write a lot of the content, which is why so many of them have an identical format.)
I agree. When I checkout of an online business and it has a space for coupon/promo code, I think it's just part of the package they bought for sales. Makes one hopeful, though!