There is this concept about burning your bridges. Being that person that just leaves without notice might catch up to you.
I had to give four months notice for a job; that is absurd. Two weeks keeps options open. Employees are also eligible for unemployment insurance, while employers don't have "cog insurance". Employees can dispute wrongful dismissal. The balance of power is much more equal than most people realize, until you get up to companies with over 5,000 employees or so.