It's because if a people exist for a long time, they will almost certainly leave behind traces of their activities: remains of their food, used tools, architectural remains, buried coffins, etc.
因為人們在地球上生活，日子久了之後，總會留下一些活動的痕跡，像是食物的殘渣、用過的工具、破壞的建築物、埋葬的棺木等。