Earlier quoted context omitted.
I believe that being a flight attendent is seen as a desirable job, and therefore they just don't need to pay more. They can get keen and well suited people to apply for any job openings they may have, regardless of what the airlines are paying.
This is true, but this same logic applies also to pilots. The key difference is that pilots are very expensive to train and therefore more difficult to replace, whereas flight attendants are relatively easy to train and replace.
http://en.wikipedia.org/wiki/Chesley_Sullenberger
You'll never seen an airline advertising how they have the youngest pilots in the industry.
On the other hand, having flight attendants to be as young, attractive and perky as possible is probably considered a positive thing.