If an employer mistreats you, why make life easier for administration by giving them a deadline to hire your replacement? You're the one who has been wronged and has had to take personal time out of your life to search for a new employer.
It's also a complete double standard. Employers don't warn employees two weeks before layoffs or termination. Why has society set the standard that workers must provide a two week notice when the same courtesy isn't offered to us?