Too many hours thrown at FF7 (chocobo farming, side quests, enemy skills, final weapons, etc)
Too many hours thrown at FF8 (triple triad, more side quests, ultima weapon, etc)
Too many hours thrown at FF9 (tetra master, even more side quests, chocographs, ozma, etc)
Too many hours thrown at FF12 (hunts/marks, too many side quests, SCREW YOU YIAZMAT, etc)
Despite the iffy stories, drawn-out combat system, somewhat questionable magic systems (looking at you Junction) and many other negatives that every individual FF game comes with (no game is perfect), it's just because there is so much to do in the old and new games that I keep playing and enjoying them.
Than again, I haven't played FF13 so I can't make a judgement on the "old vs new" debate.