[kristov]

I'd say a big part of it (aside from the valid "framework" comments) is that game engines abstract over a large and complex mess related to graphics card APIs on different platforms. That abstraction is of the type that basically forces game engines to be frameworks, as opposed to libraries. If you want your game engine to run games on Mac, Linux, Windows, Android, iOS and the web, the game engine needs to "wrap" the game. Or at least this "wrapping" makes both game engine and the games simpler to write. But as mentioned, it means switching a game between game engines isn't really realistic - you write a unity game.