The simple answer is that it's expensive to implement.
I have some ideas for how it might work, but the more stuff like this there is, the more complex the system becomes.
Here's a screenshot of as much of the upper body Animation State Machine as I could fit on the screen:
If there was a new set of animations where you were now holding the sword exclusively in your left arm, most of those would need another entry. Same if I implemented something like dual wielding.
So it's not to say that we will never do those things, just that it takes time, and it might be more awesome to do something else first.