It's very weird. You would think the people posting the jobs might know for sure, although different posters may have different motivations. It seems something that should be amenable to some journalistic investigation, but while it's getting a lot of talk between job seekers, including in these comments, nobody seems to have tried to do such investigation to get some clues as to what's going on.

The simple explanation is incompetence: the ratio of false negatives is too high, so everyone has a story of being rejected from jobs they could perfectly fulfil. The ratio of false positives is also high, so jobs remain open for longer they should.

I recently met the CTO of a unicorn in a social function, and they urged me to apply, he would fast track. I interviewed there before, got an offer but decided to go somewhere else. Even with a face-to-face invitation from the CTO and obviously having passed an interview in the past, they rejected me.

There is no end to incompetence in modern business.