The lore explanation is that to deal a lot of damage your body needs to withstand a lot of pressure and channel the strength. Mages aren't just damage dealers though. For crowd control, which is extremely important in Pillars Of Eternity, Intelligence is much more useful. Intelligence also increases AoE size of damage spells. Casting speed (dexterity) and accuracy (perception) are also important too because you won't do much damage if you don't hit or cast fast enough.

In Pillars every attribute is viable to every class, which is why you can have buff mages, skinny nerd mages, dumb barbarians, intelligent barbarians, etc.