That's probably the issue: since O'Hare has lots of custom ground vehicles, the parkings are intentionally flagged as GA, to get rid of the default ones, which slows down the fps considerably.
This means, the traffic needs to rely on the Airline code but, default traffic doesn't have it, so they park in other areas. If you check the whole airport, you will see some traffic, perhaps in the cargo area.
Since most of the users use 3rd party AI traffic package, this is not usually a problem, because the addons traffic AI always have the Airline code, so they park at the correct place.