Why do people like anything? In fact, here is another question; why is there so many "why do so many people like X" threads today? I mean seriously, I have seen 3 thus far with at least 2 pages worth of comments.
As for the topic at hand... it is like an RPG, in that in the single player survival mode, you work up from nothing, and eventually become the master of the land, diamond sword in hand. You can build almost literally anything if you have the ingenuity, time, and resources. It is also quite nice to get the satisfaction of finally finishing that one project, or starting another one. Playing with friends on a server is very fun as well. Having many people working on one huge project can be quite fun.