Depending on the job market, in situations where there are many good candidates for each position companies generally are satisficing not optimizing, and in that case simply discarding some percentage of candidates (i.e. anyone unusual) is not a drawback (since the difference between the best candidate and the next few runners-up is less than what you can measure) and so arbitrary filtering gets done.

Of course, that would usually not be an obstacle in job markets where acceptable candidates are scarce; SWE vacancies are not the same as journalist vacancies.

I suspect that employers who are more secretive themselves may find the lack of social media or other nonymous online presence as a good indication that the person in question is unlikely to be someone who could publicly share internal information accidentally.