Greeting with Good morning is same as we start our day with some kind of habit or ritual like yoga, meditation, jogging or making a cup of tea/coffee and in the same when we meet someone first time in the day we wish them good morning to start the communication and wishing wellness through out the day.
Just imagine someone you know comes and doesn't talk and how bad you feel that they ignored you, so just wishing them good morning will make them happy and creates a healthy atmosphere and makes a pleasant feeling