Aero, Aero2 And Aero3 exist - but only certain monsters use them.
I guess they didn't bother with separate materia for every element because there were simply enough materia already. Adding more would've just over-complicated battle - you might end up spending twice as long flinging spells at an enemy in an attempt to discern its weaknesses, and it'd also be rather difficult to protect your own characters against elementals because there'd be more options than materia slots.

Also, having a "holy elemental" materia would've caused significant confusion regarding Aerith's white materia.