Lil, your questions are always so insightful for your 12 years of experience, thank you for this one in particular.
The U.S. is and has been suffering under a terrible misunderstanding of its place in the educational achievement world. This misunderstanding has caused many of us to spend way too much time in handwringing about our self-described mediocrity of late. Beginning in the hyper-paranoid Reagan era of public policy wherein the National Research Council coined the document "A Nation At Risk (ANR)," and continuing through to the Common Core State Standards response to the No Child Left Behind/Every Student Succeeds Act policies we have been frantically attempting to pull ourselves up by our bootstraps. We perceived ourselves as falling behind from our former glories and therefore in need of making ourselves over yet again.
The data however does not support our misperceptions of it. The US is among the most diverse culturally, economically, and educationally populations in the world. Third largest in raw numbers behind China and India (which of these is the largest depends on which week you look), we boldly attempt to educate every single member of our population within a system originally designed to educate a very small, select subset of that population.
I could go on and on about this but will try to focus more on your question of what prevents us from adopting a more playful learning model? In a word, FEAR. The ANR told us that we had in effect waged a war of mediocrity upon ourselves and those words have scared us so badly that we continue to flounder about for more and more control of our situation...not realizing that those fearful words were flat out WRONG.
We as a culture need to come to grips with the real data that our achievements in education over time and populations is NORMAL (0,1) for one thing. Then look to other systems that are in fact achieving marginally higher in areas we would like to improve in, especially those that seem to graduate students who still enjoy learning after their 13 years of schooling.
I said this recently to a group of educator friends and have decided to make it my schooling motto, "If a child graduates from High School less curious about the world than they entered it in Kindergarten, then schooling has failed her, we have failed her." If we set this as our standard to achieve and forego the relentless drive to over assess through over governing what happens in classrooms, we will remake our schools into places of joyful experience and exploration. This is what I think you are referencing when you speak of other countries who seem to have this attitude.
Wow, sorry for being so wordy.