It's because the Airbender monks shave their heads as a part of their culture. If you notice, all the male airbenders are bald and the one female airbender that they showed had a partially shaved head.

Ahh, thanks for that, knew there must be a reason lol ooh yay emots!

You're welcome!

Kind of a weird custom really, but fanfly is right.

all the air benders act like monks, so they do their air like monks