Not saying you're wrong, I obviously need to go back and re-watch my cut-scenes, but those mobs you note are all non-Amal'jaa?

The pattern seems to be that with most of the Beast Tribes their members themselves don't need tempering because their Primal offers them something worthwhile to them in exchange for worship. Ifrit and Garuda both offer their beast tribes the thing they prize most, with the Amal'jaa that's strength. The human species don't have the same set of needs so they've got to be forced into the worship when the Primal has nothing to offer them.

That's probably how Leviathan got his cult of humans, because he actually does have something to offer them - ask a sailor in a storm if they'd pray to a distant goddess or a god they can see and Leviathan probably wins out over Llymlaen more than Limsans care to admit.

(Otherwise basically we've been abandoning Amal'jaa teenagers to be tempered? Brotherhood, you monsters!)