Why?
1) He has no memory. Amnesia is a classic sign of drinking too much and then drinking to forget that you forgot, continuing the vicious cycle.
2) He hangs out in bars. He lives in the basement of a bar in Sector 7. Stumbling distance ftw!
3) His girlfriend is a bartender. You know they trade cocktails for ... Yeah, you get the picture.
4) "Finishing Touch" Limit Break. Contrary to popular belief, for this limit break Cloud actually puts the "finishing touch" on downing a brew, at which point he lets out a massive whirlwind of a belch that plows through the enemy.
5) Barret's interchangable hand. You know Barret has a keg tap attachment with him at all times so that he can hold Cloud up and let him do keg stands.