Many elites are business travelers in a hub city. If you live in Dallas/Fort Worth area likely you fly AA; CLT you fly US. And don't forget despite what many FTs would like to believe scores of business travelers fly what their corporate travel department books them on. Alliances mean each airline can probably get you to just about anyplace in the world.
The number of heavy personal flyers only/mostly is a small percentage of the FF population (particularly at the highest level) but over represented here on FT.