but what can explain WHY Americans are failing the marshmallow test at an individual level?

Is it because of complacency, which, in turn, is based on the perceived sense of invincibility? Again, why? Or is it because the lack of the habit of self-control because of the false interpretation of unlimited personal freedom to do what ever you want to do? Again, why? Or because of the lack of any sense of responsibility (e.g., government will eventually pay off all the irresponsibly taken student loans)? What makes many, if not most, Americans so nearsighted (or plain stupid?) that they can't foresee the consequences of their actions just a few steps ahead?