tmobile will not trade in my paid JOD device towards a new device

So i recently just paid off my lease for my samsung s21 ultra and i was looking to trade it towards a new phone. But every time i tried to upgrade, it tell me that i cant trade in my phone since it is a lease device. I spoke with a rep and they told me that, i will need to wait another 9 months before i can upgrade???!!!!!! I dont understand. I finished my lease and paid the purchase option but im still being told that i can not use the phone towards a new phone since it is a lease device. Can someone please explain to me, why pay off a phone for it to technically not belong to me?