This Reddit thread refers to "stressors of flight" (many of these stressors are also present in automobiles), sudden altitude adjustment, lower cabin humidity., proximity to strangers.
Yet flight attendants face these stressors too, but they don't appear tired?
I don't think sleep answers my question. I know that on long flights, they are allowed to sleep in crew rest compartments, while passengers sleep too but still can remain tired.
This is bordering on opinion based as its a bit subjective from person to person but generally speaking flight attendants and pilots are more accustomed to the typical stressors of flight and potentially less subject to them after some time as the human body adjusts.
Perhaps more importantly this is also their day job, its what they do. When you get on a flight it may be in the evening, or early in the morning, maybe you are coming off a long work week and really burnt out or that weekend in Vegas really got to you. When you get on a flight you may likely be tired, and small, dry, noisy spaces are not where you want to be. This is not the case for the crew. They are there to do a job, presumably well rested and ready to go.
Sitting still is surprisingly tiring. Flight attendants are up and moving around for most of the flight, and that physical activity helps keep them awake.
They're also more accustomed to the lower oxygen levels and other physical and mental stresses of flight, similar to pilots, so even if the activity levels were the same, they'd be better able to handle it.
FAs receive the same kind of fatigue and circadian rhythm management training that pilots receive, and have similar statutory duty hour requirements that limit time-on-duty and require minimum rest periods between duty, and so on.
A Flight Attendant's job is quite unusual, in that the majority of their training, and their real job function, is as kind of an undercover riot control cop, who can expect to spend their entire career working their nominal role as a cabin steward without ever implementing their training except in relatively minor ways.
In addition to all the other fine answers, there is also a statistical error in your sample, more precisely in your selection criteria: you have a selection bias in your sample, because you are ignoring all those flight attendants that quit their job because they found it too tiring.
In other words: one possible reason why flight attends are not constantly tired by flying is because if they were constantly tired by flying, they wouldn't be flight attendants.
