The problem is not that most people going into job interviews are unqualified for the position. The fact is most people are qualified. If qualifications were all that mattered, employers could hire candidates from their resumes and skip interviews altogether! The difference is how well you perform in an interview and this includes answering and asking questions, testing, and behavior.
The keyword is "perform." Your interview is a performance. In addition to your answers, you are being judged by your appearance, how you dress, how you shake hands, eye contact, gestures, how you sit, how you stand, your voice inflection, body language, what you say, what you look at, and even how you drove in and exit the parking lot.
Your resume or skills can help you get in the door but they do not get you the job. The problem is most people do not have the correct answers for interview questions and have little or no idea how much their behavior is impacting their chances.
When you are going to a job interview like most candidates you usually have no idea what questions they are going to ask or what signs the interviewer is really watching to see from you. Most candidates have no idea what signals they are sending out or what the interviewer is detecting about them.
I was once naive enough to believe that if you answered all the questions correctly you had a good chance of getting the offer. (Yes I was living in a fairy tale!)