With Avantgard, it is clear that supers exist in other nations on Earth. The events creating super-powers are NOT limited to just the USA.
But, since we aren't reading about super-incidents in China (which houses roughly a third of the Earth's population), the indication is that most "super potentials" go through their lives without exhibiting any manner of super-powers whatsoever.
If I'm understanding the fluff correctly:
1) Not every human being has the potential to be super. In fact, fewer than one in a thousand have this potential.
2) Not every potential encounters a "stress event" that activates their powers. The majority of potential supers live normal lives.
3) Not every potential survives the stress event, and a significant portion suffer physical or psychological damage from the process.
4) Most superheroes are not "weaponizable", either due to lack of power level or powers unsuitable for combat.
Pure Theory:
Some manner of resident trauma (psychic residue energy) seems to be created with every super origin. This energy seems to encourage/generate/be generated by violent crime. So either superheroes and supervillains rise because of the breakdown of society, or at a certain level of chaos there are more "stress events" that generate heroes. The issue gets thornier if the USE of super-powers generates this dissonant energy.