Building things is fun whether it be virtual or in RL. You achieve a sense of accomplishment when you look at the finished product. I haven't played minecraft, but in recent years I have revived my childhood love of Legos. Building something from a set of instructions can be fun, but building something spontaneously using only your creativity and imagination can be so much more fullfilling, regardless of your age. From that perspective, I can appreciate why people enjoy minecraft.