Because the game never actually ends and the story ends up taking a back seat to grinding/buying new stuff. The entire reason I play RPGs is for the narrative, getting to the next point to see what happens, and then sitting back and watching the conclusion once I've defeated the big baddie.
If FFXI and FFXIV had a storyline with an ending, I would play them. But that will never happen because the games would lose so much profitability.