When I took business 101 in college one of the first things they taught us is that long term, fixed metrics will always become gamified, that both the ones measuring and the ones being measured will replace the real results with the metrics and sacrifice the first for the second. I understand that this is common knowledge in the administrative world. Yet, every single performance metric always becomes ossified as the only target that matters, every time. Why?
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.