At the level of industries and large groups, the chief answer to your "Why?" is the same sort of reasoning as the old "Nobody ever got fired for buying IBM": Nobody ever got fired for using established performance metrics.
On the individual level, there's another tricky problem, which is that very few individuals could figure out an alternative performance metric that beats the established one, no matter how gamified the established one is.
On the individual level, there's another tricky problem, which is that very few individuals could figure out an alternative performance metric that beats the established one, no matter how gamified the established one is.