Corki

I'm not sure how to answer this on the Miscellanious board.

On the Memes and Games board, I'd say it's for Gameplay reasons. (Or probably say nothing, since you would just be making a joke.)

On the Gameplay board, I'd say it's because the Grounded effect only prevents champions from using abilities that move the champion very rapidly or instantly. It doesn't prevent anyone from moving normally, not even champions that fly.