Your answer will change greatly depending on who you ask. My personal theory is it has to do with pacing. I personally can't find enough time to keep up with all of the things I could be doing in FFXIV. On the other hand, I also do not play as often as many people, and I tackle a lot of things at the same time; leveling Lancer, Armorer, Leathercrafter, Gladiator, and Carpenter all at the same time really feels like a juggler trying to keep one too many balls in the air.
At the same time, many people will tell you that there isn't barely any content at all, these are the people who, for better or worse, have what they want already (level 50 classes with the best armor). When each patch is released, they can jump right in and tackle what content is in that patch, clearing it fairly quickly (because let's face it, you can only add so much content at any one given time). I'm not saying they're wrong, just have a different perspective. Only you can decide which perspective you will likely see.
Starting from the ground up, the game has plenty to keep you busy for quite a while; whether you enjoy that content, if it's right for you, or if you will have an opportunity to visit all of the content, that I cannot say. I for one, quite enjoy myself, and after this last patch have several times been at war with myself over getting one more task done or putting the controller down and doing some work that I need done tomorrow.