Amex is indeed pretty aggressive with expansion of its Centurion lounges -- including as evidenced by its going increasingly into DL territory -- but sometimes that general posture is not aggressive enough to get it everywhere it would consider (and has considered). Amex could have jammed in a CL into ORD if they bought a controlling stake in an airline, but Amex is sensibly far removed from being inclined to do that. That would be too aggressive.

ORD had been aggressive about trying to lease out space and get a slice of the revenue from leased out space, but I'm not sure if getting a slice of the revenue from tenants is also applicable to lounge space leased by airlines and/or companies with a controlling stake in an airline. Does Amex give any US airport a percentage of entrance fees it collects for additional guest entrances into the CLs?