It's easy to criticize conquerors who went to war for personal fame, glory, and spoils. But for all of Caesar's, Alexander the Great's, and Richard the Lionheart's faults, you do have to give them some credit for putting their own lives on the line to accomplish their goals. Each could easily have died in combat. They always had skin in the game, even if the game wasn't particularly moral.
So I'm curious how we evolved from the standard of the heroic general/monarch/emperor leading from the front, often at the head of a cavalry charge, to one who either stays behind the lines, directing the battle through messengers, or more recently, delegates battlefield command to subordinates while staying at a safe command post miles away, perhaps sending off telegrams or orders over radio when needed.
When is a difficult question, because to a certain extent, even as far back as Antiquity we have societies where leaders absented themselves from both combat and even behind the lines battle command, while there's also have the former President of Chad, Idriss Déby dying in combat against rebels in April of 2021 supposedly from gunshot wounds. However, as for why, I can provide two reasons for monarchs: the need for military legitimacy, and the scale of the war effort.
As I wrote for a previous answer about the emergence of kingship in early medieval Europe, military legitimacy could be the factor that could make or break a reign, not only in medieval Europe, but in most regions marked by security issues and instability. From Frankish warlords on the periphery of the Roman Empire to Byzantine Emperors in the 12th century, personal military success was a strong way of securing legitimacy not only for the coercive hard power it represented, but because it also signalled that they could protect the elites of an embattled land. For example, the Gallo-Roman aristocracy transitioned pretty "seamlessly" into Frankish society as the Romanized world of Gaul collapsed. However, personal military command did bear risks, not only could a monarch be killed, but they could also be captured, which even if they survived the ordeal, could prove fatal to their future political success. Romanos IV was only a Seljuk hostage for 8 days after Mantzikert, but that was enough to trigger a series of civil wars. The risk of losing in battle was such that imperial Roman and Byzantine systems evolved to develop a system wherein it was custom for a senior officer (or the emperor) to claim credit for their subordinate's actions. In this way, the Emperor could have his cake and eat it too. Especially as political systems solidified and became gradually more impersonal, the need for a militarily legitimate monarch disappeared. They were no longer the monarch just because they fulfilled a military function, but because of the title they carried, or the family they belonged too, as well as countless other legitimating factors.
Additionally, the more wars are fought and the farther they're fought from home makes them more difficult for a monarch to personally command. The tetrarchy of Diocletian was a means of making sure there was imperial authority in all of the Empire's crisis zones, so that the response in each region could be fast and effective, and not be slowed down by the need to wait on official instructions from the Emperor. Napoleon led his own armies, but had a group of Marshals to which he could delegate important military commands. As brilliant a commander as Napoleon was, he couldn't fight in Central Europe while also putting down a revolt in Spain.
These two factors also intersect, and a good example of this is Basil II of the Byzantine Empire. Basil played a highly active role in the military campaigns of his age, which allowed him to build up his own military prestige. At the same time, there were simply too many problems for him to deal with personally. If he was campaigning in Bulgaria, and Arab raiders attacked Anatolia, he could delegate a military commander to respond. In this case, Basil tended to take on the largest campaigns and challenges himself, while still delegating as much as was feasible for him. Hope this helps!
OP I think based on this question you would enjoy the book “The Mask of Command” by John Keegan. He addresses the transition from leading from the front to leading from HQ. And it’s a great read! :)