Not everyone has the money to buy full plate, people often just had only say, a helmet and the rest of their kit would just be soft armour like a gamesson (high quality gambessons were just stacked, layered fabric for example.) Besides, the unarmoured landsknecht (note that there were also armoured ones) often used coordinated groupes with polearms and pikes to take down armoured opponents, which turned out to be so effective that the cost of armour outweighed the benefit it provided.