[creeble]

Indeed, sometimes they do and sometimes they don't.

Why doesn't the rest of the world (outside the US) have a 3rd middle brake light (CHMSL), as was mandated in the US in 1986?

My guess is because, despite measurements of ~22% crash avoidance at the time, that number turned out to be between 0% and 4% in practice. Maybe the rest of the world didn't care about 4%, or maybe they thought it was as stupid then as it turned out to be.

But the CHMSL standard in the US has refused to die.