Because you are bidding the other a good passage in future journeys and the word "good" implies that no ill feelings are harbored against the one to whom you are bidding leave.
If you don't think it's a good bye then just say bye.
Good question..But i can't really give you a good answer :/
Sure.. your wishing your friend, family member or beloved a "good" bye.. you're wishing them well after you depart... seems right to me.
Good bye is to wish someone well until you meet up with them again and that you have intentions of doing so.
Because it sure beats "Bad Bye."