>>30682015AZ spent 3000 years looking for that thing, and by the time SM happens he's only been reunited with her for 2 years.
I have no idea why they decided to make Eternal Floette an actual form with different stats and even an exclusive move when story-wise it's pretty much the hardest pokémon in existence for the player to get.
Catching gods and extradimensional creatures sounds perfectly doable compared to waiting until an immortal old man either dies or decides to give his most cherished pokémon to a random kid.