The Titan and Leviathan Primals are not in "slumber". They were all roughly summoned 8-10 years ago when the Paragons appeared and that's when the Beast tribes became aggressive. Before that they kept to themselves. We don't really see them attacking, but that doesn't mean they aren't at work in their Beastman cities, dictating what the tribes will do. In fact Leviathan we do see, in the LL storyline (but I don't know what will happen to that since Levi got redesigned.)

This new post even says that the Immortal Flames could make a lot of money by helping the other city-states with their current Primal problems, since they have experience with keeping Ifrit at bay. So obviously the Primals DO attack from time to time, and they've been doing it for years. We as players just haven't seen it.

The only Primals who haven't been summoned are for the friendly Beast tribes, like the Sylphs and the Goblins.