Minecraft is an open world game where the player is allowed to do almost anything they want. In these types of games the player can set their own objectives if they want. This is nothing new. Games like GTA and Fallout have done this for some time. The difference in Minecraft is that there is no story and the objectives are an afterthought. There is an "End" and an achievement tree but this is mostly for tutorials and usually ignored by players. Instead people set their own goals and try to achieve those goals for themselves.