My personal hypothesis is that this is related to the fact that the people who drive cultural output in the U.S. (the Left, basically) take a hands-off approach

My personal hypothesis is that this is related to the fact that the people who drive cultural output in the U.S. (the Left, basically) take a hands-off approach to transmitting culture to the next generation. As a result, each new generation in this sub-population of culture creators has had to engage in a high degree of cultural re-invention to fill in the gaps.