Because knives appeared earlier than metals such as copper and iron. Slash and burn farming, in that flood era, human beings used stone tools and wooden sticks, and they relied on grinding them into knife shapes little by little. This is a process experienced by social development, and when human beings start to use tools, they do not smelt metals, but can only use local materials. Most of them are bones, sticks, stone tools, and metal ore is roughly like stone. Therefore, knives were originally not related to metal, and most of the knives in primitive societies were mussel knives, bone knives and stone knives.
Moreover, the literal meaning of the knife is not to refer to metal, but to sharpness. Cut up quickly, this is the knife. Only later, the shape of the knife became more and more stereotyped after development. And because the metal material is much higher than the bone stone, so that the later knives are made of metal, but the society has become more and more knives. At least the plastic knife that cuts the cake is not metal.
In fact, the bronze knife appeared earlier than the earliest surviving script oracle bone on the mainland. But we can't be sure that there were no words before the Oracle. At least we have heard of Cangjie," and the word "knife" in the oracle bone is a very simple two-stroke stroke, much like a real knife. It is not known whether this was handed down before or created by oracle bones, but from the perfect system of oracle bones, which records the humanities such as farming and astronomy, it is not difficult to see that this has undergone a certain development. Therefore, the word "knife" appeared earlier than metal. And because the knife is an indispensable thing in life, and even a sign of the development of a civilized era, the word "knife" must be the first batch of words to appear.