Because the sum of us is greater than the one of us.
If you approach negotiations with an adversarial mindset (believing that it’s you against them) you can’t create more in an agreement than the separate parts of what each of you brings. You both enter into a struggle to win over the other and get the most you can for the least amount.
If you win you will be pleased - at least for now. If you lose, you will want to even the score.