It was a real phenomenon, but it’s also one that’s been exaggerated.
While peasant levies did exist and tended to be not as well equipped, they didn’t really need to be. Spear and shield and helmet will do the job most times, and once you get into longbows, then your peasant levies are actually hugely valuable without the need for plate armor or horses.
Moreover, as time went on, you started to get specialization. Professional men-at-arms who could bring the equipment and skills of a knight to the infantry were hired with cash that kings raised from taxes in lieu of military service, mercenary companies emerged to deal with the slowness of pulling together feudal armies, and so on.