When I worked at a very small company we were extremely concerned about this, and so we paid people well enough that they didn't want to leave. All I can figure is that the bean counters just don't understand that churn has a cost.

some places like Amazon operate around the churn. Keep everyone anxious and they won't try to collectively bargain nor ask for raises. They won't be around long enough anyways.

Same reason why companies don't pay everyone 10 million bucks a month. Where do you think the money comes from?

Where do you think that money is going?

Generally I understand the missing factor to be a control thing.

Th power structure that makes up a typical owners-vs-employees company demands that every employee be replacable. Denying raises & paying the cost of churn are vital to maintaining this rule. Ignoring this rule often results in e.g. one longer-tenured engineer becoming irreplacable enough to be able to act insubordinately with impunity.

A bit bleak but that's capitalism for you. Unionization, working at a smaller companies, or at employee-owned cooperatives are all alternatives to this dynamic.