This is called ellipsis in English. When the meaning of some sentence is understood without a word or two in it, we leave those words out, and this is called ellipsis. See below for some examples of ellipsis.
1- What time did he die?
(Ans) He died at two in the morning.
(Ans) At two in the morning.(More natural than the first
2- When in Rome, do as Romans do. is better than:
When you are in Rome, do as…
In your spicific example, the “we are” is understood, so the author prefers to use ellipsis.
I hope this helps.