Why isn't +Mana considered a combat stat?

I mean more mana means you can use more spells. More spells is more damage. Still having still having Mana when your opponent doesnt is a huge advantage. I'll concede that it doesn't do anything in an all in, unless you have mana for a fight when you would have been oom without it. So I'm not quite sure why +mana isn't considered a combat stat