Why does an employee stay on the job when he is not satisfied in this job?

Actually there're different reasons, for example:

• Some people accept to stay due to financial reasons (when there's high salary/benefits, or the employee has no other sources)
• The employee is waiting for another opportunity to come, in the same company or another company
• Waiting for promotion, or looking to get enough experience before leaving 

In this case ther's a simple rule 

We should wait until to seize a better opportunity