Seems like at any time in my life there's at least a few people who have jobs they hate but who aren't trying that hard to change it, and I wonder why. I've never had that problem; I've certainly been stuck in jobs I hated, but I never felt unable to be at least trying to improve my lot. So I was wondering what stops other people, and I came up with a few reasons.
I promised to stay. I'm surprised how often I hear this one. Now I'm all for the idea of giving your boss a fair shake. If you made a commitment, you should honor it. But often people are standing by this kind of promise way longer than they actually committed to, or standing by it well after the job had failed to live up to its promises to the employee. You should be as loyal to an employer as it is to you. If this is your reason, are you really just using that as an excuse?
I can't schedule interviews because of my work times. If your job is during the same hours that your prospective new employers and you can't get time off, you might be able to work around this some of the time. Ask the new employer if they can do an off-hours interview: many won't be able or willing, but some might appreciate that you're trying to honor your terms with your current employer instead of calling in sick, and respect that. Or just call in sick if that's what you have to do. Sure, you will get fewer opportunities this way and it may take longer, but that's no reason not to be trying. If anything, this is reason to try harder since it'll take longer.
I don't have time or energy to look. Sounds similar to the previous one, but in this case, it's more about not putting in the time and effort. This one is very pernicious. My job is so stressful that when I'm done with the job, I am too tired, or emotionally exhausted, or stressed, to do anything serious. All I have the energy for is to relax: watch TV, play games, sleep, whatever. I need these things to burn off the stress. But it's a self-perpetuating cycle. It may be hard to break but it's the only way anything is ever going to change. You have to buckle down and make yourself spend some time on job-searching and interviews. For some people, the best way is to schedule that time. If you know you only have to do it on Mondays and Thursdays, maybe you can make yourself do it, knowing you can crash on the other five days.
I'm not qualified for a better job. Maybe you're not qualified for a lot better, but even without qualifications there's some hope of doing better because there are better jobs out there where you can learn as you go. Usually, though, this one is an expression of lack of self-confidence. Don't decide that you can't get the job: let the interviewers decide that. It's worth a try: why not roll the dice? Maybe you're afraid of getting a job where you can't cut the mustard, and then losing it, and that's a legitimate concern. But there are certainly jobs that you can try for where the important currency isn't skills but effort.
I'm afraid a change might make things worse. An insidious one because it can be legitimate. You're making a bet when you change jobs; it could indeed end up worse. But sometimes this is an excuse. The correct comparison isn't "will that job be better than this one". The correct comparison is, "will staying in this job forever be better than the next job, given that if I get it, and it doesn't work, I can try again to get another one". People tend to compare one day to the next, instead of multiplying the effects out over many years or a lifetime. The other thing people fail to consider is that interviews don't have to be one-way. You can be interviewing them just as much as they are interviewing you. Go into the interview not just trying to wow them but also with a plan to evaluate them and decide if they're going to be a good job. If they offer it, you don't have to take it.
I get too distraught when I don't get a job. This one seems like a minor one but I think for many people it's the biggest one even if they don't realize it or admit it. Each time they go on an interview they get their hopes built up, and each time they don't get the job, their hopes are crushed, and they get that much closer to giving up. You have to remember that there's usually dozens of applicants. No one can go into an interview with any certainty of success. Every interview is a long shot; they only lead to a job in the aggregate. That's not an easy way of thinking for a lot of people. They get daydreamy about how good the job will be and then it weighs down on them. I don't know a solution for this other than "stop being like that". Remind yourself over and over that each interview is not "me trying to get a job" in itself, but only as part of an overall process.