Why no corporate sponsorship/co-branding?

Riot has a pretty big player base and I've always wonder why they haven't teamed up with different companies to offer fun skins:

Coca-Cola Volibear, Scyther Kha'zix, Kraft Cheese Twitch, 100% USDA Angus Alistar, etc,..

Why not incorporate some product skins and maybe make runes free to compensate for all the bonus $cheddar they'll make. Ayyyyyyyy?