The problem was that the powers of the Ancients that were running amok and destroying the world due to the sound. The raw power of the Ancients was working against them until the very Star was failing (in Emet's own words). Even if there were vast aether reserves held within the other life on the planet, they might have already been destroyed before the Amaurotine choose to act. For all we know, the life pre-Calamity could have been teeming with aether. But from what we see in the duty Amaurot, the Source was already a burning wasteland and Amaurot collapsing before Zodiark was summoned. There wouldn't have been many sources of aether by that point, hence why they probably resorted to human sacrifice.