Why is there still a mana cost on Kog'maw's W?

I'm genuinely curious. I don't actually think he's that bad anymore with all the new builds people in the Kogmmunity have come up with. I mean sure it'd be impactful, but not overwhelmingly so. It would certainly come in handy when the situation calls for it. I think it's certainly possible to have mana problems in the early game, especially if you're playing on the back end of the lane (Because you'll have to farm with your spells to hit CS).