It's not. Nature is not fair. School is not fair. Dating is not fair. And so on. We try to build fair societies, but right now, they are not.
And interview processes are really, really unfair. Being disappointed they are not and writing a rant about how to them more fair is very naive.
Interviews are most of the time social evaluations packed with negotiation. Very often, even the ones that pretend to be technical interviews.
Why ? Because humans are imperfect and limited. Most interviewers are just not capable of an effective recruitment process, no matter how they believe they are, not to mention a fair one.
So the advice to send here is not "make your recruitment process better". People that are able to will, and don't need this article to do so. People that can won't, and the article won't change their mind.
No, the advice is to the people looking for a job: don't take it personally, randomness takes big role in the process. But if you want to maximize your chances, you need to work on your social skills. Especially the ones that make them feel good about the interview, and value you.
You need technical skills to be good at your job. To be able to apply at high skill ones, to network with people with alike skills, to have fun, to negociate money.
But to nail the interview, you need more.