Realistically... without the stamina system, there's no particular reason you wouldn't want to guard if you could.
Then again, with the way Guard is implemented, making it automatic would be an insanely powerful defensive buff for all classes equipping 1 handed weapons.
I imagine they decided, at least for now, it was easier to leave Guard manually triggered, rather than recode how the shield system works and re-balance the defensive advantages to avoid making certain classes too durable (mages especially).