Hi Anthony! I can't answer your initial question about individuals reasons for staying in their jobs but to help get the ball rolling, I will contribute that there are basically four attitudes that can become habitual ways of approaching our work and life that may be learned responses or just infiltrate slowly due to life experiences that have established a less than positive approach.
There are those who don't like what they are doing but they continue to do it because they believe they HAVE TO. A 'have to' life position creates an energetic stance of resistance and resentment and the experience that life is hard.
Those who don't like what they are doing and avoid doing the job with any impetus often harbor a 'GUILTY' stance towards life. They don't like the job and don't want to do it but they believe they 'SHOULD' want to for whatever reasons they have told themselves. Guilt is the price we pay to hold an inaccurate image of ourselves while consistently violating it by non-performance.
The third habitual comfort attitude is to avoid going after anything we really want by procrastinating, lack of preparation, lack of decisive action, etc. which creates a 'could have, should have, would have' IF ONLY stance ( if only I had a degree, was older...younger...smarter...knew the right people etc) that sets the stage for an emotional climate of regret and further disappointment.
The last comfort zone requires a conscious choice. It is that if we take on a job, career, profession, we commit to bringing our highest and best to the environment we are participating in. This means that if we are going to do the job, we make the decision to want to do it. It doesn't mean that we only do what we want to do, instead it is that we exercise the self-discipline to direct our energies fully and clearly so that we create the experience of inner satisfaction from full participation as we pursue external results.

Reasons could be different according to persons and countries.
In some countries is challenging to find a new job, not only a well paid one including benefits, but something else in general.
Persons could not afford to quit, maybe they are scared about this solution as somehow they have set up a comfort zone (even if they don't like the job), they don't want to be considered as job hoppers (again in some countries it is normal to switch to different jobs, in other ones is not), you don't have a beautiful exit or a B plan in place.

In brief, while 80% of workers do not uniformly fit your exaggerated description of them, I am sure that there are a vast number who are dissatisfied with aspects of their work situations.

The reasons for remaining in one's unhappy work situation are highly variable and can include those you include in your headline, as well as others.

The bottom line is, people continue in unhappy or dissatisfied situations until they either decide to seek a new one or the employer makes the decision for them.