Interesting observation. The area of the upper lip is known as the "Danger triangle" of the face, in which due to way blood circulates in that area, including the upper lip, infections can spread back towards the brain:

https://en.wikipedia.org/wiki/Danger_triangle_of_the_face

I never thought that mustaches had a health / self preservation / natural selection origin, but it could very well have been the case. Shaving every day without modern tools and products could easily cause infections, which in that area could prove fatal.