Actually, many weapons were made from copper (and wood, but even modern firearms nowadays and military firearms up to round about the 1960s are partially made of wood).

As the person below mentioned, they very much had access to copper. We find quite a lot of copper artworks. We find surprisingly few copper tools. This makes sense. Pure copper kinda blows for tool making. Stone is harder and easier to get. And the problem lies with the purity. Other civilizations had to accidentally discover early metalworking and eventually stumbled upon bronze, being more durable than anything else at the time. North America, up near the great lakes actually has fairly pure raw copper deposits on the surface. A theory is that it led to the native people finding a "new" type of rock, but other than being distinct looking, it proved to be kinda useless.