I've been following this thread with great interest and I finally have decided to chime in. There are theories of how the Native Americans got here, including walking across a land mass or frozen glacier over the Bearing Straits and there is a theory that they came from Asian explorers who sailed across the ocean. The latter is starting to gain in favor. The Clovis theory I guess you would call it, says they came here around 16,000 years ago. The clovis, by the way, is an eliptical or cigar shaped stone spear point. The theory about the Asians coming over in a boat, I think says they came here about 30,000 years ago. But if you ask a Native American they will tell they have always been here. Being a decendent of Native Americans and German I prefer to think they have always been here.
There are probably as many theories to why the Native Americans did not take up forging and smelting as there are archaeoligists, however the Central and South Americans did forge and work precious metals such as gold and silver. Shoot that's why the Europeans tried to kill them off at first (got xxxx close in the West Indies), later it was because of fear and ignorance.
To answer your question, and I am not an expert, I would say they didn't forge iron or copper because it wasn't necessary. They were flourishing without it.
Wants some information on Native Americans. Google "Wounded Knee" and "Trail of Tears."
Fred