The problem, especially when it comes to more ancient artifacts, is that metal objects often got melted down and repurposed at some point over the centuries, especially during hard economic times in the middle ages. In digs, archaeologists find lots of trash, like broken pottery, but people would rarely throw away a sword with usable metal, even a broken one. Nobody would leave valuable weapons on the battlefield, either. Ancient swords are sometimes found as grave goods or in military camps, and they found some at Pompeii (Pompeii, untouched from 79 CE to the 1700s, has been a veritable trove of metal artifacts, from swords to medical tools). The one ancient sword I've seen irl was in pretty bad shape, it was very corroded, and it has since been restored. But most swords have not survived.