The reason is job dissatisfaction. Studies have endlessly shown that the majority of people loathe and hate their jobs. They attend to their jobs in quiet and utter desperation. They see work as a necessary evil and as a means to pay the rent and other bills in addition to putting food on the table and a roof over their heads. This is a constant refrain from working people everywhere. They hate their jobs but they endure as they do not see no other recourse.
Many people feel as if they are enslaved by their jobs. People have been indoctrinated from childhood that work is WORK. They also have been told that they DON'T have to like the job as long it as provides "job security" and "pay the bills." So as a result of this inculcation, people see work as a lesser evil, looking forward to breaks, lunches, leaving to go home, vacations, holidays, and ultimately retirement. They do not expect to love their jobs but to ENDURE it. If they complain about their jobs, they are told that they should be "thankful" that they have a job and working to contribute to society. They are furthermore told to "grow up" and that a job is not necessarily likeable.
However, there are smart and/or brave people out there who do not like their jobs and instant of desperate acceptance and complaining, they do the INTELLIGENT thing and quit such odious jobs. They realize that being in a job that they do not like is toxic to their mental and overall health. They further believe that life is too short to work in something that they do not like and/or are passionate about. They want to work in a field and/or area that they love and are passionate about. As Dr. Wayne Dyer stated do not die while your music is still in you. So many people meander through their work lives, doing something they hate/dislike, and realize much TOO LATE that they could have been doing something else.
However, I am disgressing. Many people, even in these precarious socioeconomic times, quit odious jobs because they refuse to proverbially be somewhere that they do not like. They refuse to let socioeconomic situations and circumstances deter them from achieving their respective career/job goals. They also do not want to be like the rest of the rank and file and accept any job because according to the socioeconomic dictum, "it is better to have ANY job than no job at all." They KNOW and WANT better for themselves!