I don't have any insight into what deals the carriers have on the backend, but in my region, the price delta between like-for-like BYOD plans and plans with subsidized iPhones are effectively nil.
Either the carriers are getting discounts from Apple, or everyone with BYOD plans are subsidizing phones for people who get them through carriers.
e.g. I'm currently seeing a carrier plan where the total plan + device cost over 24 months is ~$1,600 and you come out of it free and clear with a 512gb iPhone Air that's $1,749 retail. I don't see how that's possible for the carrier to offer if they're not getting discounts from Apple.
My point is that Apple doesn't want to look like they're giving discounts on their store, so they engineer any discounts to go through third parties.