This is something I've questioned repeatedly myself; the closest answer I've found so far is that most here seem to have either been involved in a major event - affecting either the world in general or just a sufficient number of people, whatever "sufficient" may happen to be in their world's case (the criteria seems to be different based on the state of their world in general) - or, if they weren't involved, they at least witnessed it or stood to be affected by it in some way.
Basically, an impact was made and this world noticed.
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Basically, an impact was made and this world noticed.