One Year's Notice?

Aren’t you supposed to just say "I quit" and leave?

So now they are saying that Simon Cowell is going to leave American Idol next year.  I am so sick of people leaving, but how come they give so much notice?  Oprah gave like a year's notice too it's pretty ridiculous.  You are supposed to go suddenly and that really gets people talking.  One day you will turn on Ms Chelsea's show and I just won't be there and it'll cause a huge stir!  Then I will come right back because I really like my job.  Ms Chelsea is just so-so, though.