You can install new apps onto android from other stores like amazon or download straight from the web. Fortnite just did the latter by avoiding the play store

You can install new apps onto android from other stores like amazon or download straight from the web. Fortnite just did the latter by avoiding the play store and selling their app direct from their website. For IOS though, fortnite launched it in the IOS store because there is no real practical way to get users to install their app outside of Apples app store.