It's a flaw in his design paradigm. Ideally, he's a late-game hypercarry with an abusable early game to make up for it. In reality, his kit makes that impossible. His kit is fantastic at zoning out melees, giving him a strong early game on top of a strong late. If you try to take away his strong early, it means you've pretty much gutted his abilities to the point of uselessness. If you try to take away his strong late... you've probably also gutted his abilities to the point of uselessness, because his abilities are so spammy. As a mage, his role is to blow people up. If he can't do that, he can't do his job, and thus isn't played. But his kit means that when he can do his job, he's ridiculously efficient at it, while also being fairly tanky (prior to the rework, because of his build, but now more because of the shield his passive grants). A root every couple seconds isn't exactly the paragon of good design.