Google spends absurd amounts of money developing and supporting Android, providing security updates, providing feature updates, and even goes so far as to spend money marketing Android. They then give Android, for free, to manufacturers to use on their phones. They also create, support, and give, for free, many of the development tools which developers like Epic Games then use to create Android applications and games.

After spending an insane amount of money creating and supporting these things, it seems completely justifiable that Google would then ask for a cut of the profits for applications that are built on top of the tools and infrastructure that Google provides for free.