Brook in his last two fights showed how tough he was. He can take a punch, get hurt, then keep on throwing. He is tough but isn't stupid. I've seen way too many of these tough fighters that are just dumb.

When you are getting beat badly and taking a ton of punishment, you don't owe anybody anything by continuing to take punishment just to prove that you won't get KO'd. Brook had lost. He knew that he really didn't have enough to win the fight at that point and it wasn't worth permanently damaging his eye.