Why serfs become knights?

It was highly unlikely for serfs to become knights. Serfs were bound to the land by law, and were considered "villeins" with few legal rights. Their primary role was to farm the lord's land, and they were beholden to the lord for protection and provision. Knights, on the other hand, were members of the nobility, typically sons of knights or other noblemen, who were trained and dubbed as knights through a formal process of apprenticeship and initiation.