Stone and ceramic are very durable (the latter tends to break into potsherds, but doesn't rust away as iron does). And population density doesn't need to be particularly high to be able to locate settlements and dig up artifacts.
This is simply not true. There are many places where you can go where the impacti of the Native Americans is still very obvious. There are places where you can find hundreds of religious carvings, arrowheads, obsidian artifacts, ect.