I agree with the OP - it's extraordinary just how bad the FAs are on American carriers. Years of flying UA has inured me to the awfulness but, as my wife says, even the grumpiest crew on BA or VS are charm personified compared to the best UA crews - and I'm reasonably confident that the same comment would apply to AA or DL ones. The best UA crews are competent and force a smile, but are charmless. The worst are like prison wardens.
But why? Blaming the unions is not the reason as so many other airlines are also unionised. Age is certainly an issue but perhaps tenure is more the culprit here - years of doing the same job in deteriorating surroundings will take its toll, but other airlines face similar issues.
Certainly relentless pay and benefit cuts must be significant. I remember flying UA some years ago during the BA FA strike over pay. The UA FA told me that the BA FAs were, pre-strike, earning over double the UA London-based crew. If you go into a job in your early 20s that is high status and well paid and, 30 or 40 years later it's the opposite - and you are still doing the same thing - it must be demoralising.
But essentially I think it's a management issue. Management does tend to get the staff it deserves.