The Theory of Aether

[Enkidoh]

Well, aether is essentially energy so therefore it possess no will of it's own. It's an outside force that manipulates that into a form for either good or ill. Primals specifically, only come about when an Ascian specifically instructs someone exactly [i[how[/i] to create a wish into reality by summoning (Gilgamesh summoning my namesake doesn't really count however, as the whole Hildibrand questline is firmly wedged tongue in cheek and isn't really truly canon to the wider storyline, and thus that whole quest should be taken with a grain of salt).

This is the crucial point here - simply having a boatload of crystals and a strong will is not enough to make your wishes manifest as reality, there is a specific technique known only to the Ascians which is the missing piece of the puzzle. What that technique exactly involves is anyone's guess though, but what is clear is that it's essential to make summon a Primal or Primal-like entity (this is why Eorzea isn't filled to the brim with people making their wishes come true simply because they have a strong will and lots of crystals ), and that it is a secret that only the Ascians have knowledge of - hence why they appear to the downtrodden or defeated members of society, whether the beastmen or spoken. They appear in a non-threatening form promising them 'salvation' and the ability to make their dreams reality, instructing them how to summon with crystals.

Additionally, it's interesting that Ysyale never once mentioned exactly how she becomes Shiva, she holds that secret close to her chest even after we gain her trust and helps us. Whatever it is, it's the smoking gun so to speak.