Minecraft isn't just a game anymore. It is a cultural touchstone. An entire generation has been introduced to video games through Minecraft.

Furthermore, the gameplay effectively can go on forever. Especially with mods on PC, there are very few limits to what can be done. Entire separate games (e.g. Hunger Games mod) have been implemented.

Also, it's one of the best-selling games of all time. That helps. No one bats an eye when, say, Puyo Puyo Tetris gets released and ported. (Yes, I just compared Minecraft to Tetris.)