Because the game doesn't assume facing. It assumes you're constantly turning and twisting, facing one direction one moment another the next.Agreed. Why does everything need to be "square" anyway? Can't we have a monster which "grid-size" reads 4x2?
Size could be listed as "Huge (4x2)".
If you did the 4x2, you could easily trap a creature facing one direction without the ability to turn 90 degrees.