I thought ok, why not, I bought it about two months ago. I wanted to see what was all the fuss about. I don't like it, the only "fun" I had was surviving (and I'm using the word loosely) for the first day and night cycle via my first shelter. Then, I got bored. I never played in since the past two months or so, I literally played it for about two days total. As to why is it not finished? Who knows really, there's plenty of other games out there released as "final" and "completed" but once you play them it yells "BETA!" at you constantly. I mean honestly, most gamers don't know how games are made and what should be considered final or beta or whatever (finished or not), they buy a game and they expect to have fun, if it's not fun then they can say whatever they feel will justify the boredom coming out of the game, calling it unfinished or unpolished or uncared for.
Let's take Dragon Age 2 for example (dead horse I know, but it's one of the most recent and well known example I know of by heart), do most gamers consider the game "final" and completed? Well... I don't think I need to say anything else then. So yeah, Minecraft may well always be in a "beta" state of development and people will buy it for various reasons, be it to "support" the creator (even though the guy is now a few times millionaire) or to "give it a try". The same goes with Dragon Age 2 and many other games out there that end up on the store shelves and labeled as completed by just being on the shelves to start with, even though it was obviously rushed and still is most likely beta.
I just think that generally most gamers don't really care anymore if a game is completed or not, if it's released and the price is good for them or the game "seems fun" they'll just buy it blindly and call it a night, complaints usually follow after but at that point it's too late, their money is in the publisher and developers' pockets.