Many reasons.
Games take you away from reality as you take role of a particular character, therefore becomes entertaining.
Games have puzzles and learning curves on how to play (sadly most of them lack of it now), therefore becomes entertaining.
Games allow you to control what you do, whatever you do, dictates on what happens, therefore becomes entertaining.
Games allows socialising, such as multiplayer, you compete your skill and wits against each other, therefore becomes entertaining.
Games provide a challenge for you, and serve as a distraction, as said above about it taking you away from reality, therefore becomes entertaining.
I doubt there's any other reason to play it, apart from of those reasons.
Fuck yeah, repeat the same ending line "Therefore becomes entertaining".