Because there’s no icon for it
By now, many of us are familiar with Google TV, the interface layer that Google introduced with last year’s Chromecast and later spread to other Android TV units. The homescreen is focused on recommending shows and movies for you, relegating some of your apps along with app search and discovery to a secondary tab. A major side effect is the lack of proper access to the full Play Store, even though the app is still there.
How to access the full Play Store on the Chromecast with Google TV was written by the awesome team at Android Police.