Because there was no such thing as "the nation"

The core thing to understand about medieval rulership is that it was built on persons and dynasties, not institutions. We are very used to the idea of the "nation state", where we insert rulers and officials into institutions that are usually organised and regulated by a constitution, and those institutions are how we conceive of the state. We think of "the government" or "the [insert leader name] administration", and that government is not divisible through succession because constitutions and institutions ultimately transcend the people in charge. This was not a feature of the medieval kingdom.

Medieval rulership did not have many institutions. They tended to have taxmen and judges to administer laws, but that was about as far as it went. The actual ownership of the land and associated titles were usually in the hands of individuals, not institutions (an exception being land owned by religious institutions such as monasteries). This meant that when an individual ruler died, the land and titles they owned were theirs to pass along as they pleased. Out of a desire to be fair to their children and avoid conflict between them, many rulers decided it would be best to divide their titles and lands when they died. For example, William the Conqueror thought his eldest son was unsuitable to be a king, so he gave the title of King of England to another son of his, William Rufus, but gave his eldest son Robert the consolation prize of the Duke of Normandy. The Conqueror thought this would be fair, though Robert disagreed. This was commonplace where rulers had multiple ambitious successors, though it often resulted in conflict between the heirs as they attempted to seize all of the previous rulers' land and titles.

A notable exception is the Capetian dynasty of France. The indivisibility of their domains was a key reason for the growth and success of French royal power in the Middle Ages. Early Capetian monarchs would crown their favoured heir while they were both still alive, elevating the son to the status of co-ruler. So when the elder monarch died, there was no succession that could divide the royal domains because the new elder monarch had already been a king for years. Eventually they stopped this practise once the idea of primogeniture succession (eldest son inherits everything) had become entrenched.

Another notable exception is the case of thirteenth century England. Although the continental possessions of the King of England continued to be the personal possessions of the people who ruled them, England itself began to take on a more institutionalised character in the twelfth century with the establishment of the Exchequer and of Parliament in the thirteenth century. This was largely because English kings were far more concerned with affairs on the continent (meaning war with France) and it became necessary for England to become essentially self-governing in order to harvest tax and pay the royal army while the king was otherwise engaged. In the 1150s, a philosopher and English government bureaucrat named John of Salisbury argued that England should transition from a royal domain to what was known as a Res Publica, and proposed a model of government in which the autocratic power of kingship still existed, but where the business of government was handled by institutions made up of upstanding and educated bureaucrats. Being an upstanding bureaucrat himself, and closely involved with the business of the Exchequer, he realised that the Exchequer could quite happily plod along with its business while the king did whatever they liked elsewhere. When Richard the Lionheart became king and expressed little interest in governing England and spent almost every year of his rule waging military campaigns, that became a reality as the Exchequer and a regency council ruled the country. It was an unwelcome shock when John took over and started getting actively involved in government. Then, when John's son Henry III proved to be pretty bad at governing, Parliament tried to overthrow him led by the nobleman Simon de Montfort. This "Res Publica of England" or "Commons of England" as it was sometimes called, was styled as a constitutional monarchy and attempted to establish a coherent constitution with the Provisions of Oxford and Provisions of Westminster, but Henry regained power a few years later and abolished them after it became clear that Simon de Montfort was more interested in taking England for himself than governing a Res Publica. Even with an appetite to do away with the idea of royal domain and move toward something that we would recognise as more of a "nation", it proved impossible to get away from the idea of England as a possession of one man.

So that's why. Medieval kingdoms were fundamentally thought of as being personal possessions rather than abstract concepts or collections of institutions bound by a constitution. There were people who thought that was dumb and wanted to nation-build, but they were only ever a minority and failed to gain traction outside of the odd, temporary coup.