Why do hunting and gathering societies still exist?

Why shouldn't they? That is, what would force them to no longer exist? What I mean is, something would have to happen to either destroy them all, or force the people in them into other societies. No such thing has happened, that's why they still exist.

The people in hunting and gathering societies were born into them, and are living the way they were raised to live, the way their parents, and their parents, and so on have lived for a really long time.