How MMOs traditionally keep you involved for long periods of time performing otherwise dull, repetitive, and tedious tasks is by programming the player.
That grind you were so fond of is the classic way you were programmed to be addicted. It honestly had nothing to do with skill, or even "content", but a stimulation of the reward centers of your brain.
Going from level 1 to level 2 is quick and easy. You don't have many skills to use and mobs practically impale themselves when you attack them. Then level 2 hits and you learn a new ability. Using this new ability makes the job of killing easier, but the mobs you started off attacking at level 1 are less rewarding. Also, the road to level 3 is much longer, requiring more XP. So you go off to fight harder foes to improve the rewards and reach level 3 faster.
Once you do you get that satisfying Level Up, you get more slot points, more points in your attributes so you can kill even faster. But your current foes are now less rewarding, and the journey to level 4 is even longer. But! At level 4 you can learn a new ability and get even stronger, and so you press on with harder foes. As you level, you collect drops which you can trade for money, which in turn, you can exchange for gear, which makes you stronger, and allows you to earn rewards faster.
And it's by this process of making the combat experience increasingly more complicated and rewards gradually more infrequent that you're being conditioned. It's also reinforced by how the game tracks your achievements. Your character has a page on the Lodestone that gives you these trophies every time you reach a new Aetheryte crystal, reach a leveling milestone, complete a challenging quest, or kill an exponential amount of mobs. You can also use your character data to create a fancy signature image so you can give your friends an idea of how much you've accomplished in the game. Lots of classes sitting at 50 makes you feel respected, important, like a real player.
Powerleveling does circumvent this conditioning somewhat since it creates a shortcut around the process. So while you feel this may create players who are less skillful (somewhat incorrect given that there is some skill involved in setting up an efficient powerleveling operation), what you're actually dealing with are players who are less addicted because they haven't gone through the same process to endgame that you did (and therefore don't love the game as much as you do). Because they can easily obtain signatures filled with high level classes, your similar achievement seems less important. The reinforcement of your achievement is thus shattered, and your addiction to the game is not as strong.
However, the greatest force of addiction is not really in scheduled rewards, such as the leveling system, in the first place.
The greatest addiction occurs when the rewards are random.
The Ifrit fight is actually an example of the most addictive part of FFXIV. You know that if you fight Ifrit long enough, you'll eventually get that highly valuable item you're hoping for. But you don't know if it will take 1 attempt or 100. Not only that, but you also know that not fighting him guarantees you won't get the item (can't be obtained through trade or alternative methods). The same goes for creating materia. After a lengthy process of filling the Spiritbond gauge for materia creation, you actually don't know if you'll get the highly desired form you're hoping for (this time). And let's not get started on the process of doubling them up when inserting them into gear...
It's the perfect conditions to keep you playing over and over, flailing at the chance to stimulate the reward center of your brain, knowing that if you do nothing you won't succeed, but that next time, maybe next time, you'll get that which you truly (were conditioned to) desire.