Why is a sprite^2 the Ultimate Self? Is there an explanation beyond "it just is, because an Ultimate Self was needed for the plot"?

It was really not even needed for the plot, but it was a really good thing to talk about. Sprites draw from Skaia as a source of knowledge, but are somewhat limited by being a Construct of sorts. When stacked upon another Sprite, however, it seems these limitations are just unlocked, and as such the Sprite is able to access a deeper well of knowledge, specifically, about itself, and what its made of. Davepeta mentions this would likely be kind of hard for a normal being to take, but Sprites are made for that sort of knowledge, so to my understanding, it just gives access to Sprites to even more knowledge than they had before, and the closer you are to understanding reality, the closer you are to understanding yourself, and vice-versa.