Maybe Obsidian held its edge better.
My tiny brain, armed with no systematic knowledge of physical anthropology, has thought about that question in general, ie. why no metal tools on a continent abundant with metals? My answer is that perhaps they lacked a ceramic pottery hard enough to melt metals in. And even if they could melt it, or just hammer soft metals like lead, copper and gold into tools, they may not have been as good for the job as things like bone and obsidian, as Sawdring suggests.
For another reason, I believe a stable civilization with large, permanent cities is prerequisite for something as advanced as metalurgy. Most NA peoples were semi-nomadic, following the herds. Time to think and experiment are luxuries of advanced societies.