Because demoting and merely kicking layabout pirates isn't very satisfying. Keel Hauling would immediately distribute ALL resources of the offending pirate to the other members of the guild. Huh? Huh? It's good. Right!?
There were a couple of suggestions a while back that would satisfy your frustration with unruly members The Captain could impose an automatic Guild Tax on all members of the Guild to help pay for perks - thus helping out all members; not just those active and responsible. Another satisfying suggestion was the 'black spot' placed on an annoying member when kicked - the result would be a 1 month block from re-trying to enter that guild again; and a visible mark on his/her profile for all other Captains to see for the same period.