The game engines are sort of like the operating system for the game.
Think of all the Java programs and such out there. Java would be the engine all that software is written on and needs to execute the program.
Game engines are the software used by the game makers to make the game. The largest studios have the talent and funds to create their own engines to add exclusivity to their software, and the engine itself can be sold for other companies to use to make games.
An example would be Ubisoft and their Snowdrop engine which was used for The Division.
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