Their friends don’t know about it.
When you have a girlfriend or a boyfriend, you feel ecstatic for finding that right person. You tell your closest friends about them and then introduce your partner to your friends. SInce you are only a backup plan for them, they only introduce you as a friend. They don’t want people to know that you are together since that would affect their chances of finding another person. Their friends would assume it is a commitment so they want to avoid the fuss and are fine with keeping everything a secret.