Um... mages do exactly ONE thing.... cast spells. They're all about that shit. Their autos are crap. They're usually very squishy. A lot of them are immobile. ALL they do is throw spells at people. Because of this, they build mana regen... buy mana regen items, use mana regen masteries/runes, and generally have more mana to play with. Without mana they're usless.
A tank's job is not to die. They are meant to soak up damage, not dish it out. Very few of them build any kind of mana regen items, and most of them would be kinda broken if they could spam their abilities. Galio probably needs his mana costs spiked, but they probably left them lowish because he used to be a mid-lane "mage", so he still has some similar traits to one.
More often than not, low mana on a tank isn't a problem. They're not supposed to be spamming spells or winning lane, usually. They're a supportive role meant to place passively early and then be useful as a front-line in teamfights. None of this requires mana. Tanks are not mages. They are not splitpushers who need to spam spells on the minions. They are not duelists who are expected to fight alone. Now, if you think the role is lousy and dislike it because it can't spam spells and be more pro-active... that's your opinion and it's valid. But tanks have enough mana to do what they need to... and they need very little. If you wanna be more aggressive and use more spells, play some bruisers or mages man.