People can quit their job for many reasons.
I left my previous job only for one reason. The reason was simple. I worked in retail, we were extremely shorthanded and the boss started taking 3-4 hour lunch breaks and showing up an hour or two late(often intoxicated or made no effort whatsoever to sleep for his shift). The rest of the employees were adopting his attitude towards work and taking extended breaks and an overall poor work ethic. Upper management refused to fire him, and they would never fire me because I was shouldering all of the work. So, either I had to bear the workload of 2-3 people, all while getting no recognition for it whatsoever, or walk out.