Although the notice period has long been seen as the only way to proceed when an employee wants to leave a job, it isn't like it has to be a requirement. If anything, it is a show of respect to a company that quite a lot of them do not actually deserve. Management can get rid of an employee without any warning and without giving them time to formulate a backup plan, after all.
The fact is that both parties are needed in the employer-employee relationship, and an employee has more free will in certain situations than employers would like them to know about. There is only so much they can do if somebody wants to quit and not tell them about it. Frustrating as it can be from a manager's perspective, it is one of the reasons that they get the bigger paycheck. Also, it can often tell you something about what kind of work environment is being cultivated. The only issue is that the type of people who cause sudden walkouts are not usually the type of people to self-reflect in this way.