Because they all knew that if they showed their weakness now, it meant that they had admitted defeat. How could Jack, who had always been arrogant, admit defeat?
"Why are you still unwilling to release your energy? You have such a powerful force, and it is used in battle, isn't it? You are still protecting these abilities, and you can't be my opponent at all."
When Zeus was fighting, he didn't care about Jack at all. He had released all his energy, which could be recovered in an instant.
This made Jack feel more difficult to deal with, but he still used another tactic to deal with Zeus.
Seeing that Jack couldn't deal with him anymore, Zeus was even happier. He thought that Jack was no match for him at all.
Now he could seize this opportunity to humiliate Jack as much as he wanted. Jack had heard what he said, but he didn't have much reaction.
He was trying to find a way to take advantage of Zeus's weakness or to release all the people in the cage.