Because all we have to go on are some murals, a few suppositions, and the word of Emet-Selch himself. The two possibilities we may infer thus far are:

1.) Zodiark had to be resummoned after fulfilling its original task.
2.) Zodiark did not have to be resummoned, but it did not have enough power left after its initial task to restore life and habitability to the ruined landscape.

Nothing we've seen up until this point suggests that he started demanding sacrifices in exchange for the tasks that were given to him.