What was the basis for the independence of the large feudal lords in relation to the king?

In the Middle Ages, feudal fragmentation was observed on the territory of almost all European countries. Which is characterized by the weakening of the unified power of the ruler and the strengthening of the power of the feudal lords on the ground. This was due to the fact that the economic independence of the feudal lands grew, which led to the emergence of strong armies. Everything needed was produced locally, subsistence farming was developed. The nobles no longer wanted to depend on the royal power, since the feudal lords were often stronger than the king.