Because his last name is Beard?
Anyone know why the drummer in ZZ Top is the only one without a beard?
Maybe it's what Hippy said.
Or maybe the other members grown beard in honor of their friend drummer and his unique last name! So he's Mr.Beard already! Doesn't need beard on his face!
He said in one interview with Howard Stern because he didn't have one he is able to keep some sense of anonymity. I suspect there maybe more to it. Such as he either doesn't care for the style or it just gets in the way. Not only drumming but also in ranching.
Not everyone can grow a full beard. Perhaps he does not have the facial hair capable of growing a full beard. I happen to be of those that can, so I do, but my wife won't let me grow a ZZ top type beard.
He's like me, special and different ;)