Catchup
OK sorry for my vitriol.
However I think there's an easy answer:
None of them can afford it. If they could, the last 747 order from an US passenger carrier would've been more recent than January 2001.
The US carriers are going after the point-to-point model - hence they are looking to replace their old point-to-point aircraft (767s).
It's simple money and passenger volume.
TD67