The Vegas fanfest had a small story about why Eorzea (and the rest of Aldenard is so aether-rich). I wish I could find the exact text, but the general tale was that once, all the continents were together. The other continents we bigger and bullied Aldenard, and became jealous of its gifts, moving away. The story was about how the continents split apart and why there's distance between them. Because the flow of the aetherstream is at Silvertear Lake, Aldenard naturally has the highest concentration of aether. The other continents are further away, which lessens the flow.
