Because he is not the Alliance as a whole. Even Saurfang acknowledged this in saying that there would be no peaceful resolution this time and the Alliance would have their pound of flesh at the end of A Good War. The characters in the story don't know that Anduin is a radiant god who no one in the Alliance will defy and who will bring peace and love if we just let him. No, what they know is they started this war with genocide and people like Genn and Tyrande are rightly pissed at the Horde as a whole. Compromise or surrender at this stage would be the end of the Horde. These same people were in a room not along having Varian inform them of this in clear terms.
Baine and Saurfang should be laughed out of the park in-story for suggesting surrender and for hoping their own side loses and more die to appease their ego respectively.
One of Saurfang's most consistent traits is his complete abdication from any responsibility. He spares Anduin, but he also doesn't warn Anduin of the trap that he views as so dishonorable it's worth quitting the Horde over. A lot of apologists forget this, but Saurfang didn't even quit because of the Blight, he quit because the idea of blowing up an evacuated city was ideologically offensive to him. He wants to get rid of Sylvanas and bring 'his' Horde back, whatever that is, but he doesn't want to do anything until the God-King grants him freedom.