Well, you don't need to believe it, just to know it is true. Luck is a concept that easily describes people's misfortune. It would be unnecessary long to really go into detail as saying 1 in 50,000 LSD users will get hppd, because that person is susceptible to it, since he has Gaba or NMDA receptor dysfunction or whatever, which landed him with hppd.
People on this forum are really the extremely unlucky folks that took drugs. There isn't a statistic to really point how many people develop hppd, but from how rare doctors encounter it, you might assume that it is rare. I mean, only Dr. Abraham, one guy on the whole planet is studying this? And he is doing this since the 80s, now that's sad and unfortunate to any hppders out there.