Simple. It's the result of a game design error. There is no collision in the air and orbital layer, it's simply disabled. Distance between Anchors and these extractors is ONLY enforced client side. Remove these restrictions in your client by editing the json files, and the server will happily stack them for you. For any layer where collision is enabled, the server would never allow such thing.
Would you look at that - it already seems fixed (at least client-side) as of today's patch (v. 72996). What a pity... it was so much fun to stack them :<
