Mostly certain people just kind of got the Sight.
It was rare. Very rare. Its appearance didn’t even necessarily follow any hard and fast rules or patterns, either, though there may have been a correlation between its development and surprisingly close supernatural experiences during, after, or even before the Transcendence. It also came (comes) in different strengths and flavors, so it would be extremely hard to document as well, and as time went on people found that it might be inherited in family lines, or else it might vanish entirely or appear out of nowhere instead.
Basically it is and was practically impossible to predict.
