There are a lot of answers to your question but I guess the shortest one is : Freedom and lifestyle.
You are falling prey to exactly what airlines want you to think about flight attendants. "waitresses in the sky". Did you know that years ago they required a Bachelors at the then best airlines for a while? Guess why they withdrew it? They were getting killed in contract negotiations. I admit that at US it's probably around 60-40% that have a degree, given that a lot of US f/as originated in PIT, which is a blue collar town. Still, if you looked at Continental, United, American and Delta I would guess the percentage is higher. The more educated you are, the more likely you are to be curious about the world and look for an affordable way to see it.
I do have experience outside the airlines, I worked for a fortune 500 company. I hated every day, seeing the same people, knowing their worlds were small and mine was getting that way. Education buys you choices, and in my case freedom. It is my belief that every flight attendant should have a plan B, it will make us stronger. So do I use my education? Sometimes, sometimes not, but my world is bigger than most peoples' and I like it that way.
Open your mind, just because a guy is an aircraft mechanic doesn't mean he doesn't read Proust. Maybe he just likes to work with his hands and not take work home. Just because a flight attendant pours drinks doesn't mean they don't have a rich personal life. The flight attendants that you see that are bitter and mean, they don't have a life outside this nonsense. Some of them may not have a degree, but they're learning more travelling than the jokers in the sandcastle with their degrees who have made US the worst airline in the world.