Most job seekers think that the entire job search revolves around them. In fact, there are often many reasons why someone doesn't get the job that have nothing to do with experience, skills or ability. Below, I've listed some of the ways that the hiring process can go sideways for a job seeker
Is the organization listing a job that has already been filled? It sounds dumb, but sometimes corporate policies or regulations require a company to advertise a position that is no longer available. It happens. This is a classic reason why you don't get the job, but it's not the only one. Here are a few more:

Is it just a headhunter trolling for resumes? A not-so-well-kept secret in the headhunting business is that there are unscrupulous headhunters out there who run fake ads just to collect resumes. Needless to say, if you respond to one of these ads, there will be no job waiting for you.

Is there just too much going on at the company? Has a project you were working on ever been delayed because of a reorganization, budget cut or layoff? Throw in a hiring freeze and there are many reasons the hiring process can move like molasses. It seems obvious that once a company starts looking for an employee, they should carry it to fruition. Logic, however, sometimes gets lost in the workplace.

Is the company simply inefficient? Some companies are like the movie "The Gang That Couldn't Shoot Straight." There's nothing that this kind of company doesn't turn into a laborious, time-consuming and energy-draining ordeal.

Are there more qualified applicants than you realized? "Ringer" is sports terminology for a really good player who suddenly shows up out of nowhere. And sometimes that can happen for a job opening — a ridiculously qualified candidate sneaks past everyone to get the job.

Is the company being rude? Unfortunately, it's rare that a company takes the time to notify job applicants when a position has already been filled. I've argued in the past that I don't think it's too much to ask to send applicants an e-mail or postcard. Sadly, I think I'm in the minority here.

Sometimes the planets just don't align for a particular position. That's why it's so important that you don't get too bummed out if you don't get the job.