@Riot, why does Xin Zhao even have Mana?

No, seriously, think about it. This has always confused me...

Xin Zhao's Q: He stabs people with his spear, harder. On the third attack, he knocks his target into the air with the butt of his spear. Xin Zhao's W: He stabs people with his spear, faster. On the third attack, he knocks his target over the head with his spear. Xin Zhao's E: He jumps at people and stabs them with his spear. People he stabs with his spear slow down (possibly because they just got stabbed with a spear). Xin Zhao's R: He swings his spear and people close by get knocked back and stunned.

None of those actions particularly seem like they use magical energy. So why does Xin Zhao need mana in the first place?

Meanwhile, we've got manaless humans like Riven who sends big green energy blasts from her magical sword, Garen who summons a giant light sword from the sky, Katarina who regularly teleports all over the place, Tryndamere who somehow becomes immortal every one and a half minutes, etc.

Like, wtf... I mean, even if I could accept that Xin Zhao somehow needs mana lore-wise (his lore has nothing to indicate that this is the case), it still doesn't make sense in-game. Both for the nature of his skills or even their cost. Xin Zhao doesn't even have mana problems beyond maybe the first jungle clear!

Xin Zhao having mana just plain doesn't make sense to me. Why not just make him a manaless champion?