Are we set in cultural stone?

In reading Diamond's Guns, Germs, and Steel, I am reminded that we moved across the planet filling it up, one group moving on settling new territory separating from the old group, creating new cultures, new languages, new ways to live as humans. One humanity branching out from the same trunk. When we try to preserve culture, as Appiah describes in the chapter, Whose Culture Is It, Anyway?, what is it that we are trying to preserve? Are cultures set in stone? Cultures evolve. Many things commonplace in a culture today were unthinkable in that culture a few generations ago. 

Cultural tectonic plates are moving against each other as never before, often creating earthquakes in reaction to the friction. One reaction has been to cling to the practices and mores of the known in the worst Burkean tradition. Often it's more than clinging to the known, it's reverting to some imagined past, or a fundamentalist view. 

Why do so many try to set cultures in stone when they are ever changing? We seem to have an innate tendency to put people into cultural, racial, and religious boxes forgetting that we are all humans.