Well... since the mineral machines should be bringing the minerals to the knights working in the core (according to the game storyline) the machines should be only in one place. If they were to be added to guild halls, the obvious price would be high.

Or, it could be because OOO still wants people to go into the arcade. I mean think about it. You can actually access the Arcade from the "Missions" button. No other real reason to visit that part of haven, sadly :(