Because there aren't any, really. The OP hit the nail on the head: AA flies rattling ghetto sardine cans on all but a handful of routes, and somehow a bunch of people have deluded themselves into thinking that means "premium".

I picked US when I moved to my current location, largely because their hubs were convenient and they flew comfortable planes. But it looks like the merger is going to push me onto duct-taped-together Mad Dogs connecting in the well-known luxury premium world city of Fort Worth, or cram me into disgusting RJs into ORD.

And I'm apparently supposed to welcome this, and a worthless piece of crAAp of a frequent-flier program, as a huge improvement over actual customer service, actual benefits, actual planes with actual mainline service and an actual airline instead of a couple of nice birds and a bunch of run-down old junkheaps that make the 1970s NYC subway look wonderful by comparison.