Well, one could argue pornography deserves to also be on the shelves with the rest of the movies.

Stores ultimately choose what is and isn't on their shelves, and it is usually what will sell more, as well. Stores don't stock all E games simply because there is no space. So not only would a game need to pass a store's ethical test, it would also be expected to sell a bit.

Things that are rated higher actually sell less. Just like how there aren't too many R rated horror movies anymore; but their PG-13 counterparts are abundant. The PG-13 movies sell more tickets, so movie companies tend to try to make the movies PG-13.