Why do airline need so many flight attendants?

Safety. The sole reason flight attendants are carried is to ensure passenger safety--seat belts fastened, medical care in emergencies (the first attendants had to be nurses!), and especially assistance getting passengers off the plane in the case of an accident or landing problem. There are rules as to the ratio between passengers and attendants with bigger planes requiring more. The waitress function was added early, as airlines tried to alleviate passenger boredom, but that is not why the attendants are on planes.

Becasue the FAA mandates that there must be at least one flight attendant for every 50 seats on the plane. For safety reasons. And although it sometimes can be wasteful if the planes are not full, it is the law. Airlines are also trying to use smaller regional jets that have less seats and therefore require less flight attendants.

There are minimum numbers of flight attendants for each aircraft based on how many passengers, how many exits, etc. It's not about how busy they are or aren't in the the air.

The FAA requires a minimum number of staff to assist in the event of emergency, based on capacity of plane. If they are there, they might as well sell stuff.

Flying waitresses who know how to get your butt safely off a plane in case of emergency. Assuming you (and they)survive said emergency