When I shave my mustache off I look much younger than I am and I lose all credibility (not that I have much anyway).
The most probable reason is that hirsute trumpet players are deathly afraid of cutting their lip(s) while shaving and, provided the growth pattern of their facial hair is suitable for working with a mouthpiece, prefer dealing with hair than with cuts and nicks (or ingrown hairs... OUCH).
The alternate reason would be as John suggests... their wives/girlfriends/lovers prefer it. Maybe Alex, Ingrid, Lara or other ladies will chime in!
