Probably because it provides a better visual for the reader;but more so
it's both a visual and tactile thing I think. Smooth underarms almost seem, to scream vunerable. I also imagine for the armpit lovers hair gets in the way and doesn't feel good on the lips or tongue. I personally perfer no hair under my arms. I am crazy ticklish there and in spite of that, I have to admit that the glide of a feather, the carress of lips, the sweep of a tongue and of course softly tickling fingertips there is simply sublime. Sensations I fear and crave with equal intensity.
As a sidebar, after a stay in the hospital, nature had taken its course and I did have hair there. The touching of just the hair there was a different sensation and it tickled. . .alot!