If we look at the classical era and certain time periods within it, you find great people that normally appear once every hundred years almost everywhere.
For example at the end of the Roman republic you had:
- Caesar
- Pompey the Great
- Cicero
- Cato
- Crassus
- Augustus (Emperor)
- Agrippa (General)
and around 350 bc in Greece you had:
- Demosthenes (Orator)
- Alexander the Great
- Philip II (Father of Alexander)
- Aristotle
- Plato
You would normally be lucky to just have one of these people in an era, but in those times they were everywhere.
Are these people shaped into success by the times they live in (chaos in the examples) or were they coincidentally all born around the same time?
The underlying 'gist' of your post hinges on "Great Man History", so I think that it would be useful to point you towards this Monday Methods by /u/commiespaceinvader but with a number of interesting comments from others as well (Commie also wrote this related thread. Other past threads include this one with commentary from /u/LordHussyPants, among others, and finally this thread, which has less in-depth exploration of the topic, but I would quote from /u/restricteddata's response there as I think it particularly relevant for what you are asking here:
The Great Man theory of history is not that individuals mattered but that the big trends in history were due to the actions and beliefs of individuals. Which is to say, to say that individuals sometimes matter a lot does not mean one is advocating a Great Man theory of history. A more balanced view would suggest that structural developments create environments into which powerful individuals sometimes do hold a lot of sway, and sometimes they do not.
Which is to say, your question kind of places things out of order. It isn't that a number of exceptional "Great People" (because really, it isn't just men, of course!) are all born at the same time, it is that certain points in time are well remembered, and the notable personalities of that period who influenced events are remembered along with it. Caesar existed at a point where the Republic was weakening, and played a key role in its further decline, but had he not, it is likely another ambitious Roman would have played a similar part, even if in a somewhat different precise way. But then the next "Caesar", who had all the same traits and ambitions that allowed Caesar to thrive when he did, was perhaps born and lived in stable, thriving place and time, so didn't have the same opportunities.
So anyways, you'll find very few historians who would still say that 'history is nothing but the biography of Great Men'. The above links are all good places to check out, and I'm sure others will have things to weigh in with here, so hope that helps.