Simply put, the original Allagan Empire existed in a much more Aether Rich world seeing as how it had not suffered through nearly as many cataclysms as the one we now reside within. The Allagan Summoners didn't so much summon Primals as they did imprison them so that they could use their essences after imprisoning them for their own Summonings. In fact based on what is known of the Allagan Empire, the Summoners of that period were probably facing a crisis of various free-roaming Primals that had naturally come into existence of their own accord instead of by being summoned. It actually makes a great deal of sense that the very first Primals to exist were Roaming Primals much like Odin currently is.