All cultures are constantly changing and evolving - however not all cultures change at the same rate. In general the older a culture is the slower it is to change - so that over time older cultures become very stable and resistant to change. This is known as the principle of cultural stabilization. Mass media - the mass media is important because this is the way in which popular culture is transmitted through music, tv, films and advertising. Then mass production of goods allows us to purchase identical products across the country.
Malls are an even better example.
So subdivisions in California and Arkansas and North Carolina all look the same. In many places the old traditional culture has bled over into popular culture - this tends to be a regional phenomenon. As an example - it is still possible to find traditional southern foods in fast food chains in the south. Volunteer organizations etc. However, voluntary groups can also be national or international.