Someone mentioned in an above comment that managers look out for their own interests, just like employees. There are very few people in a company who are really acting in the best interest of the company.

So my take is that the reason companies behave irrationally with regard to talent retention is that no one "owns" that problem. Same reason any number of other important issues fall through the cracks at any given company.

And yes, I think you were spot on when you said that companies don't even know who they should be trying to retain.