One of the reasons. I'd wager this is what appeals to people not merely playing but mastering a particular game - playing higher difficulties, 100% completion,

One of the reasons. I'd wager this is what appeals to people not merely playing but mastering a particular game - playing higher difficulties, 100% completion, and so on. The other reasons would be overcoming other humans (esports/pvp multiplayer), discovery (story driven and exploratory games), and just passing the time (casual games).