Lots of reasons. Can't be bothered to shave....and shaving brings me out in spots
Michealangelo's rendition of God on the ceiling of the Systine Chapel has a bearded god and if we are made in his image thats good enough for me. Not sure why Adam doesn't have one perhaps he is too young or works for Disney?
Its a minor rebellion thing too......Lots of big corporations wouldn't employ bearded men as beards were seen as something to hide behind.....so I have a protest beard
If beards became popular I might have to shave mine off

Some beards look plain silly though and I don't understand the mustache without a beard or vice-versa. I just got some US currency and looking through the notes Lincolns beard is bad and Grants is good (Franklin needs a haircut)
Would Bush look more sensible with a beard or is that asking too much?