Because Zenith doesn't know you.

[ And that's just simple logic. Yuber could very well be war itself as he claims, but the fact remains that, until he can prove himself, he's just another rando talking big. ]

Regardless of your vast experience or powers you claim to possess, it is all meaningless until we see it in action. Trust must be earned, and your worth to the cause must be proved like anyone else's. As long as you can do that, surely cooperation will be no problem.