I completely agree with you, but looking at this from the perspective of a middle manager, especially a lower ranking one, rather than a shareholder or senior l

I completely agree with you, but looking at this from the perspective of a middle manager, especially a lower ranking one, rather than a shareholder or senior leadership view, it starts to make sense and isn't quite as cynical. A single IC or low ranking manager leaving a large or middle sized company will probably have no effect on the company ROI, but could seriously affect that managers goals. Outside of that the manager has a personal relationship and perhaps sees themself as a mentor to the employee that leaves, so they might take it as a personal loss even if they entirely understand the reason for departure.