Why do we have to say Good Morning at the workplace?

Iam not a morning person, never was, never will be. Why do we have to greet others with Good Morning? Why is it considered rude to say hello or just smile or nod? I rather someone just smile or not even look my way. Sounds childish yes but im serious…alot of the time I want to just be able to walk into work and go straight to my desk but I end up having to respond to others when I dont even feel like it. and most of the time you can tell they don’t want to say it either…whats wrong with “fuck off” Id love that greeting!