Why don't flight attendants get tired when passengers can, despite sleeping?
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.