Not only that, but it makes it easy for those "millions of users" to put onto their phones. Have you ever tried installing an app from a downloaded APK from your PC? Not necessarily easy for the average smartphone user. Whereas Google play allows you to install an app with quite literally one click. Additionally, it provides at least some sense of security, in the app itself from being able to download apps from a "verified" source, and as a reputable third party payment system. As many others have mentioned, the less your CC number is out there the better.

Not sure if all that is worth a 30% cut, but regardless, I don't generally go out of my way to download apps that aren't on the Play Store