Why? Well as any fraternity brother trying to justify his 1.5 GPA will tell you, it's not about what you know, it's about who you know. It's all about networking. Workers who regularly visit bars are able to build better relationships with coworkers and make all sorts of contacts outside their company. The guy you're singing karaoke with tonight is the same supervisor who might be giving out promotions six months from now, or the client you'll be trying to sell to next week.
"You're a terrible doctor, Steve, but you're a hell of a good time. How's head of surgery sound?"
On the other hand, sober working drones may be seen as good workers, but are less likely to make these close personal relationships. And with as many as four out of five promotions being based on those kind of relationships, building that camaraderie with coworkers is more likely to get you that promotion than, say, constantly working. And nothing builds closer relationships like being a wing man for the boss at a bar.
And yes, we're referring to our hypothetical drinker as a male for a reason -- the data shows this trend is only applicable to men, as no correlation between bar visits and paycheck size was found for women.