It is certainly possible through focus stacking.  If that feature actually isn't available, it's entirely opinion as to why it hasn't been added unless they have some disclosure of that, but the same focus stacking technique that works for traditional images would work for a light-field shot.

If I had to guess, I'd say it is because the entire point of a light field camera is that it gives depth of field blur but still allows shifting.  If someone want's an image entirely in focus, there's not much reason to use a light-field camera since you don't gain anything by its use, but you give up a whole lot of resolution.  

That said, I can also see the argument that you might want to simultaneously have an image that could have different things in focus and a copy of the image with everything in focus, so I don't necessarily think that is good reasoning.