They have nowhere else to go. The reason they hate teaching is because of two things. Intellectually they are not up to the job. And almost always, they lack creativity. They teach facts...badly as well, double whammy! They NEVER inspire students to want to learn.
When you go into their classrooms, there is an air of depression.
Watch out if you are a relief teacher! The students will always get ou of their boxes and misbehave.