Because he's already being paid at least what he's worth, and likely more, on the open market. If you trade for Sale, you still have to pay that contract. Think for a moment what gives a player "trade value". It's not just their ability in a vacuum. It's how their projected ability compares to the contract cost relative to years of team control. The more "underpaid" a player is, the more trade value they have. It's not controversial, it's common sense. Would you say Sale is vastly underpaid going forward? Perhaps you think he is, the vast majority would disagree.