You might be interested in some earlier threads about this topic on here. Most recent might be this one by u/RustedBarrelKing, who discusses why this change happens, but also notes that a head of state was killed in battle as recently as last year. There are also some older answers about how this type of generalship worked in general by u/MI13, in Ancient Greece by u/Iphikrates, and in the Napoleonic Wars by u/DonaldFDraper. There are also two threads about the end of this tradition in Europe, here by u/Badgerfest and others and here by a now-deleted user.