Any idea why AA hasn't tried ORD-EZE/GRU?
I know that UA tried ORD-EZE, and I think AA tried ORD-EZE. Apparently, there simply isn't enough demand out of ORD and the Great Lakes region for service to some of the major, albeit distant. cities in South America.
Demand is low and as a connecting hub for Asia/Europe, ORD is simply too far north. ATL/IAH/DFW are better located for connections.
A hub is meant to serve a geographic region. So, if you are in the Southwest and need to fly to Europe, DFW and IAH are the better hubs. If you live in the Carolinas, ATL and CLT make sense. If you live in the Upper Midwest or the Upper Great Plains, then it's ORD, DTW, or MSP.
By the same token, ORD-LHR is about 7:45 flying time, while ATL-LHR is about 8:10 flying time. If I were flying out of some place about halfway between ORD and ATL, such as BNA or SDF, ORD is probably the quicker hub to connect for western Europe.
In my mind, ORD, IAH, and DFW are too far east to be great hubs for Asia. The hubs that make the most sense are YVR, SEA, SFO, and LAX. But, if you live in places like GRB, SAT, BHM, or IND, you probably don't have a lot of choices for non-stops to the West Coast. So, DL., UA, and AA all need hubs in the middle of the country.
Now, if you remember when CP was part of Oneworld, the plan was that AA and CP would connect a lot of major cities to YVR, scheduled for connections on CP to and from Asia. CP had and extensive Asia network that rivaled UA's network from SFO, LAX, and ORD, and NW's network from SEA and MSP. When CP was taken over by AC, that left AA with very little in the way of an Asia presence.