Explain why military affairs in the Middle Ages was the main occupation of the feudal lords?

The roots of the feudal lords went to the squads of the early Middle Ages, and their main occupation was the defense of the territory with the dependent population and attacks on neighbors in order to obtain tribute. The feudal lords ruled the dependent population, received tribute from it, which could be called corvee, quitrent, or something else, depending on the specific state.

They were not engaged in trade, since they were strongly tied to the territory, and merchants were often travelers, for example, Marco Polo.