AAtrox Question, Pls Halp!

Is there a reason why Aatrox skills all cost a % of his hp while the other champs with hp cost on skills have just flat amounts that they are eventually able to outsacle with lv's and items. Some of them even have more healing returns like and meanwhile aatrox is killing himself at all stages of the game.

I built a wits end and a tri force once, worst idea ever. my hp could not keep up with the cost of blood price. i need a ton of lifesteal to keep up with his cost, and that new death's dance item even though its recommended to him his v slash and ult are magic so your still better off with a gunblade if you really need to heal off of ability damage.

im not calling anyone op or up; I just dont get why the cost of his spells are all %