The dry shampoo use is another way of not washing your hair too often, and this practice is nothing new. The idea of a daily shower was virtually unheard of 100 years ago. It wasn't until the early 20th century, with the growth of media that people started to be concerned about personal hygiene.
Scientifically speaking the hair that grows on the exterior of your scalp is dead. It contains no nerves, muscles or blood and when cut it does not hurt.