Hell, in Africa, we have the bushmen of the Kalahari who still lead a stone-age hunter/gatherer existence despite having frequent contact with their more civilized neighbors. IIRC, there are still tribes of pygmies in the central parts of Africa that haven't made the leap from the stone age yet either.
In the latter example, that's a choice. They choose to live a hunter\gatherer lifestyle.
I suspect that is the case in the former example too. I doubt seriously they had never had contact with the outside world. They chose to minimize it.
You have to be careful with this one, as the Tasadays (the prototypical tribe of this kind) were discovered to be a hoax. A better example, familiar to all of us, would be the American Indian. The vast majority of tribes remained hunter-gatherers right up until...well, as long as we let them.
You can't make the assumption that a life of cities and agriculture is happier than a nomadic life. If there's enough land to sustain you, why would you want to settle in one spot? And that's the rub. Agriculture developed first where there wasn't enough land to sustain a nomadic existence for the population.
Of course, when the Magic Book tells people that the giant grownup in the sky created the earth a few thousand years ago, you aren't going to get anywhere with argument.