Minecraft might not have the graphics of a "high end" game, but I like to compare it somehow to Colonization (old one), Civilization or Anno 1404 in terms of replayability.
With every world, you face new dungeons, new mountains, new areas and you can discover a lot of things. You can keep things simple (at least for most of the time), but you will find out things can get rather complicated (I just say: redstone and tracks).
Now, it is also possible to tinker without the survival part (using not critters and mods to get stuff) and build a world you want, with railways, towers and whatever you like.
Its not "play once, play never again". Its simple to understand, but still holds a lot of challenges after that.
I am a MMORPG veteran. I first started playing Asheron's Call in 2000, ran a large and successful guild, and eventually moved to WoW in 2005, where I again ran a large and successful guild.
For all of the good things I experienced in MMORPG's, the one thing I truly wanted was freedom. Freedom to go anywhere, do whatever I wanted, and still remain competitive and relevant.
In AC, you could spend all day running from one side of the world to the other. In fact, I organized several guild 'marathons', and it was amazingly fun. That sense of freedom was fantastic. In WoW, I raided, I ran instances, I PVP'd, but the most fun I had was taking a group of friends into Blackrock Depths, doing what we called 'Chaos Runs', where healing was useless to us - we pulled large groups, beat the tar out of them, and pressed forward without resting. That sense of freedom? Priceless.
The rest of the time, I'm going to be honest, WAS fun. Until it became obvious that each new expansion was just the same old stuff, tossed into a pretty package. It became a chore to be competitive - a chore to stay relevant. A game shouldn't be work, and when it becomes work, it's time to move on.
You asked: "Why Minecraft?"
The answer is simple, and has been stated numerous times in this thread, alone.
I can do whatever I desire, build whatever I want, waste time splashing around in a waterfall, punch pigs into a pool of lava. The best part? I am still competitive. I am still relevant.
Now, the icing on the cake is the world generator. No matter how many times I 'reroll' for a new experience, it is, without question, different. It's new, It's unique. And stated early in this thread, it's MY WORLD. NO ONE ELSE gets to play in it, unless I let them. NO ONE ELSE will create the same world that I have.
And that freedom, that uniqueness, makes this game worth it.