If all you can get from the Aether is raw elements, that could serve to explain. Pure, loose elements are kinda difficult to use. Energy states are all wrong, etc. Fine for gasses, they are loose atoms anyway (or bond on their own) but for more complex substances (especially proteins) the effort and energy required to synthesize them would make it quite difficult to use anyway. The energy calculation could still favour local 'real world' extraction and production, so you dont quite reach "post scarcity" for common goods.
Terraformer cant pull it out because it cant REACH the TN materials. There's a planet in the way. It can only reach the higher/lighter materials. Asteroid miners work the same way but they have a long pipe attached and are configured differently to target the heavier elements. Same basic principle as a planetside mine, same way a fridge compressor and a vaccuum cleaner work on the same principles.