I'm a bit surprised that anybody is surprised by this lack of success, that seems like a hugely complicated task given the limited amount of time and resources. The amount of possible scenarios the AI needs to be able to handle (fall damage, environmental hazards, block types behaving differently, lighting, day/night cycles and of course enemies) plus the extremely open-ended gameplay where you can almost always dig in every possible direction while collecting and crafting potentially hundreds of items seems to make it a very arduous task.