Because you are oversimplifying it. If the buying team wants negotiating an extension with the player as part of the deal, it is smart for the trading team to try to persuade the player to at least discuss it. Player gets paid, team gets player long term, team gets more for their asset. I don’t know what’s hard to understand. Win, win, win.
If the player has his heart set on going UFA to another team and refuses any extension discussion, that’s his right and it’s back to a one year UFA rental-level ask by the seller.