Why can you summon every elemental but Air?

"Because when you do he just vanishes in thin air..."

I agree that we should have each elemental be summonable.

The Air elemental is 10 times better than the rest. I think it's a balance issue.

If you wonder why, look at its weapon.  Its damage scales up, which means it remains useful at any point of the game.  Just like crow demons.

We've been asking for air elementals for some time.  Meanwhile, look for an update to the Goetia mod, being done by Seanw and Heavenfall.  That will include plenty of summons.

You can get a scroll or item that will allow you to summon one for one fight.