They are a part of the female body that has the tradition of always being shaven. Most body parts that can be shaved (in both men and women) ultimately come down to preference, a choice. Not armpits. Armpits therefore become this sort of mythical sacred ground that is both pure, and imperfect at the same time for reasons we already know. A place often depicted as flawless, so much that an alien wouldn't know that hair grows in a girl's pits unless someone told him, because based on what the media / ads say, you would never guess otherwise.

This contrast in perception is highly attractive, even more when you factor in that the average girl feels insecure about this body area. They must look good if you are wearing something like a top and they must smell nice because it'd be a crime if they didn't. That insecurity causes embarrassment, and that embarrassment is seen as cute by many. Last but not least, they are a highly sensitive area for both tickling and other purposes. They are not meant to be used sexually, yet they remain mostly hidden, much like actual parts of their body that are often tied with sexuality.

If you were a girl, would you show your pits for no reason? especially if you are not confident they look perfect? Even if they did, you don't normally see people flashing them. They are there but also not there at the same time. It's a part that isn't meant to be sexual yet it's always covered / unseen. Isn't that wonderful?