@Feeder "Decapitation strikes" are something modern militaries like to try too, they just use strike aircraft and missiles for it and often just have to settle for demolishing the communications infra instead because they can't actually locate the enemy commanders. Back when warfare boiled down to hitting the other guy over the head the means were naturally that much more straightforward, and for example Medieval battle accounts sometimes mention volunteer "headhunter" squads who swore to take down the enemy monarch/other leader or die trying (the latter being what usually happened; bigshots surrounded themselves with handpicked elite bodyguards for a reason).
The local top leader going down was Kind of a Big Deal in no small part because even when they weren't the direct overlord and/or paymaster of the army it tended to be their personal authority and prestige that kept the whole army together and pointed in roughly the same direction - them getting offed or captured was fairly universally the cue for the troops to start legging it, and even in the best cases (ie. the subordinate commanders could hold things together and managed to conceal the loss until after battle) severely disrupted the chain of command such as it now was.
Bigger still if he was a ruler or some similar political bigwig ofc since that could spell the end of an entire dynasty, if not realm, and usually meant some kind of succession trouble, but that wasn't of immediate tactical relevance. Sometimes the enemy leader's death was even undesirable for the victor for dynastic-strategic reasons, eg. Louis II of Hungary dying at Mohács in 1526 was quite a problem for the victorious Ottoman Sultan; he could thus not be turned into a however-reluctant vassal and instead his kingdom was then inherited by the already-formidable Habsburgs - the two empires would then go on butting heads in the Balkans and Central Europe until both eventually collapsed in the First World War.
Big Alex incidentally *himself* almost got literally decapitated at Granicus when, being the young hothead he was at the time, he outpaced the wedge of the
hetairoi and took on the three Persian satraps present solo (the trio were themselves apparently specifically going after him) - he managed to drop two but was stunned by an axe blow his helmet only just stopped and Spithridates, the satrap of Lydia, was a moment away from ending his career and life when his bodyguard Cleitus arrived at literally the last second and cut off the Persian's raised sword arm.
Make what you will of the fact alternate-history fiction never seems to make use of the glaringly obvious divergence potential here, but I digress.