The barbell effect, the smiling curve, and the r/k-selection dichotomy show up for the same reason.
There are two stable strategies at the extreme.
Everything in between slides down the slope towards one of the extremes.
R-selected systems are open-ended.
K-selected systems are close-ended.
The benefits of the two types of systems are powerful, and more powerful when in juxtaposition to the other.
That's why the world tends to strirate in interleaving layers of r- and k-selected systems.
The world pushes from mush to posterized, fractal, swirling interleaving of open and closed systems.