Because my time is valuable, and I don't enjoy harvesting. X minutes harvesting + Y minutes crafting = A gil net profit. Y minutes crafting + Z minutes buying stuff off the boards = B gil net profit. Unless A > B by a significant amount, I ain't harvesting squat.
Also, if I can buy the one stupid item I need to finish a crafting log entry off the board, and it's not too much more expensive than teleporting to the city that carries that item, buying it off the NPC, and then teleporting back, I'll happily buy it off the board.
