why can i not sell a phone for cash

Apple's provides a Trade-in program, not a Buyback program. Trade-in gives you a credit for a new device. If you are looking for cash for your device, then you will need to sell it on your own. The good news is that you are likely to get more money selling it yourself then you would trading it in. Make sure you Factory reset your device before selling.

Apple offers trade-ins for products or for gift cards. There are many ways to sell iPhones for cash including a number of reputable 3rd party online sites.