Life at the bottom rung of medieval society is not easy or healthy. In times of war peasants are forced from their farms and drafted into levies, and if they are lucky, given some kind of simple weapon to fight with. Unsurprisingly, peasants are undisciplined troops who are better at fleeing than they are fighting.
These troops are recruited from the slum of the Persian Empire, the peasants who are given a spear, uniform, and shield and told to fight. It is easy to assemble a large army of conscripts, but in no way useful.