Why are so many game developers opposed to gamification?

"Gamification" and "bringing game mechanics to serious endeavors" are not necessarily the same thing.

It's hard to talk for all the game developers, but I will give you my take on this issue:

1. The most basic characteristic about games is they are intrinsically motivated activities which challenge players in a specific skill (I am not talking about other types of play, which are not games by definition). Games are a voluntary activity - because when they are not, they aren't games anymore, they are a chore, a task, a job.
2. Most of the other extrinsic motivation-creating game mechanics (that include badges, levels, coins, etc.) are not unique to games. Moreover, they are used by organisations for decades (or more?) to generate (extrinsic) motivation (just think about schools and workplaces). I admit that the game industry perfected these extrinsic motivations better than others, as these mechanisms became critical for prolonging user-retention, although its effect on retention is still lower than introducing real challenge (and new content) into the game.
3. Therefore, Gamification brings nothing new. On the one hand, adding extrinsic motivation to real world activities won't transform them into games (which the word implies, even if it is not the exact defintion). And on the other hand, people have been using these tools to motivate other people for decades without calling them by a cool buzzword.

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