How come whenever I interview, the interviewer is so picky and judgmental and I never get hired, yet everywhere I go there are people who are awful at their jobs? How do these people get hired in?
A friend of mine does a lot of interviewing for people at somewhat high levels. Every time he fills a position it's a million-dollar decision - figuring the person's salary, benefit, overhead costs, and the amount of time they expect the person to be there. Then there's the amount of time it took to get the position posted, the amount of time they spend interviewing candidates (most of whom are qualified for the position), and the amount of time until the person starts to become useful to the company. Of course he's picky - a lot of money rides on his decisions.
From the sounds of it, you're not looking for a job at that same level. But there are still some similarities.
There are likely several qualified candidates for the positions you're applying for. The fact that you were called in for an interview means that you made the first (and maybe second) reviews. As did everyone else they're interviewing. The interviewer is trying to pick the best person from a batch of qualified candidates. Of course they're going to be picky.
But the interviewer is doing more than just seeing if you know your stuff. They are also trying to figure out how well you will fit into the company, if you'll be a good member of the team, if they can count on you to show up on time and work a full day, if you can work on your own or will need constant supervision, if you have any initiative or imagination in your work, if you're generally truthful or BSing everyone, how you present yourself and if you'd be a good "ambassador" for the company if you meet with customers or clients or partners, and much more.
It could be that one reason you're not getting hired is because you're put off by the interviewers who are doing their jobs the way they're supposed to - maybe you're taking it personally, or getting irritated by questions that you don't understand or something - and that you don't hide it very well.
You are trying to convince a company to give you a job and to pay you. They are doing you a favor by interviewing you, and a bigger favor if they hire you. They do not owe you a job - and, for that matter, you are not owed a job by anybody. If you're not approaching the interview from that perspective then your attitude might be what's costing you jobs.
Finally - don't worry about the others and whether or not they're qualified. They are not your concern. Your concern is to land a job. Once you're working for the company and have been there long enough to get to know everyone...that's when you can start complaining about them.