In this case, most of their earnings are from tips, their salary isn't enough to earn a living. Just because I didn't see the cabin attendant doesn't mean that you don't tip them. They always leave a card with a number to get a hold of them, if there is something you need done, etc.
Some of the major cruise lines have them automatically taken from your sign/sail account, to assure they get the tips earned. I do say earned, because I believe they work long hard hours to make our cruise vacation a good one.