The gatekeepers of the top scientific journals are people who themselves failed to publish in those journals when they were in academia. Had they been able to do so, they would today be authors, not professional editors. Could it be that the best science produced in the world today is being judged by the worst scientists? If true, that would be very unsettling.
It is quite doubtful that anyone would start graduate school or postdoctoral training with the idea in mind to become a journal editor. The vast majority of graduates initiating postdoctoral studies do so with an intent to become principal investigators. Something happens along the way. Disenchantment with an active career in science? Too few positions available? Harsh competition? Family choices? Perhaps all of the above.