While not focused on exactly the same topic, this HMW video has some intersection and gets into the nitty of what’s going on.

It also means the employers hosed themselves, since skilled labor is not threatened by a potential firing (the way they once were) and are continuously looking for their next rung on the ladder elsewhere. Also, it means promotions are no longer an incentive for hard work, since the workers expect you to hire from elsewhere. So all those incentives to overperform are gone.