Here's why:
* Formal singular: "Vous" is also used formally to address one person, similar to "you" in English when speaking to someone you don't know well or are showing respect to. However, in this context, it's still grammatically considered plural.
* Formal plural: When addressing multiple people, "vous" is always used, even if it's a group of friends or family.
The singular form for "you" in French is "tu", which is used in informal situations with close friends and family.