Why are companies willing to spend so much on hiring new employees but on retaining them?

On /r/datascience, there recently was a discussion “Why are companies willing to spend so much on hiring new employees but on retaining them?”. And the top reply is pretty good. I thought I know the answer but this reply showed me a new perspective. Highly recommend: https://www.reddit.com/r/datascience/comments/uo589a/comment/i8cut6w/

Because HR departments are self-serving and they generally ignore the value of productivity, knowledge, or performance in order to highlight the cost savings generated by their strategies.

Sometimes I feel like the whole IT industry (or the whole capitalism, let’s go big, just look at the current stock market) is pretty bizzare and consists of bad decisions caused by one-sided effectiveness metrics.