Because they can be anti-paladins, and therefore mages. Sure, Giant rangers can cast some spells, but anti-paladins have sleeks and are clearly more magey. I think some of the latest anti-paladin spells come from more of a pact with evil outsiders, but a non-mage couldn't handle them, and the lot of them are pure magic anyway.

I always found it appealing that one of the very dimmest races, even among the giants, can brute force their way through the doors to some of the most powerful magic around, and overcome their obvious limitations with sheer willpower.

Even the most heroic Fire Giants have an INT score approximating only the human average, but they are capable of being wise. A level one Fire Giant is something like 100 years-old, so I like to imagine the best of them learn a lot in that time, the hard way, with hundreds of years left to experience.

I like to think that this experience, their personal intensity, and that the anti-paladin class almost mirrors them philosophically is what allows them to claw out their place as the only real Giant mages.

There's a place for really intellectual, or sneakier anti-paladins, but I feel the class would be thematically poorer without Fire Giants. And if my character is going to be smart I'd rather just be a necromancer.