There's been research done (I know, big words -- don't have any links readily available), where it was pointing to the fact that what goes easily in (into the brain) also goes easily out, and conversely, what the brain requires effort and literal calories and energy to understand, stays there longer (investment must be rewarded / recouped somehow).

So while there's no doubt that facilitating learning is a net-positive, at some point it becomes a net-positive, I suppose -- your brain just chucks it out faster because it knows you can re-obtain the same information again since it worked so easily the first time. It doesn't know the difference between easy and hard, all it knows is how much effort it takes and how much reward (hormones) was generated for it all (to cement the habit/result).