How do people hack iPhone games so easily? I have seen high scores of 999,999  on Flappy Bird as well as many other impossible high scores on games such as Temple Run, etc., on iPhone and iPad.

How do people hack these games so easily when they cannot see the source code?  It seems like they know the source code of the game, which makes them able to manipulate the game file so easily.  I don't understand how this happens given that the Apple iPhone is sandboxed, unlike Android.
iOS applications store their data in files that are available from a computer in two easily-accessible ways:


Inside an iTunes backup
Via a MobileDevice-framework-based tool such as iExplorer


In my experience, game save data is stored in a fairly ordinary property list file that is manipulable via the plutil tool.  The data can be restored and replaced without any code knowledge.  From there it's a simple matter of how much the app trusts its save data.

For an example of how this can be used not just for high scores, but to cheat at a game, see the Nimble Quest iOS Trainer, which is itself MobileDevice-framework-based.

(The MobileDevice framework is an Apple private framework created for iOS device access, which has been reverse-engineered to enable the creation of iOS device tools not available from Apple.)