Imagine that you're heading out to a restaurant for dinner and getting a nice plate of nachos. However, at the start of your meal, you pull out a big bag of tortilla chips. You then tell the waiter that you've brought your own chips and don't want to use the restaurant's chips. I've known a couple of friendly restaurant owners who would agree to something like this (e.g. for allergies), but not one of them would have given a discount for it. The meal comes with chips, even if you don't want them or eat them. You're going to pay for their chips, even if you bring your own.
Most of the US phone carriers have the same attitude towards phones. Your cellular contract comes with a phone. If you want to bring your own phone, that's fine, but the price is going to be the same whether you use their phone or your own.
Thankfully, some of the smaller carriers are moving away from this attitude, but bringing your own phone can still be a major pain. After all, the only thing better for a salesman's commission than selling you an expensive contract in exchange for a phone is selling you that same contract and not giving you a phone.