Why won't you move on?

I’ve noticed that some people are very unhappy in their jobs and as a result they get sick a lot from the stress of having to go to a place that makes them miserable. Now some people may not have many options if they live in a small town but why do some insist on not applying for a job that has an enviroment or work mates that are more suitable to who they are? Is it they don’t realize that being miserable can make you sick? What are some really good reasons for a person not to move on? And are they good enough?