Why do French speakers say “allez” to one person?

Allez looks like the vous form of aller, but as an encouragement it is frozen in place: you use it on a child, a friend, a whole team, or yourself. That is why allez, viens is ordinary French despite apparently mixing vous and tu — allez has stopped telling anyone to go anywhere. Its job is to hurry someone along: allez, lève-toi !

The same frozen allez closes conversations — allez, salut !, allez, bonne journée — where it works like “right then,” signalling that you are wrapping up. It is also the shout from the stands: allez les Bleus ! Keep it apart from the literal instruction allez-y (“go ahead”) and its informal twin vas-y.