I have never understood it. Is it a form a bullying? A way to feel superior? A personality disfunction? Why do people do it? Isn’t everything better with more people around? Are some people just so simple-minded that more than one person around confuses them?
As an adult you would think the immaturity of it all would stop. That people care more about others feelings and well being. Leaving a friend alone on the holidays or excluding someone from a social event is so hurtful.
Yes, you would say someone who does that is not your friend. You are right. It still hurts.
Always reach out, it is kind. Include everyone, enjoy everyone, and share your friendship everywhere.