Why is the subject "we"? Isn't it better to use "you"?

Rebecca

The subject is "we" here because she is including everyone in the thought, assuming that everyone else is thinking the same thing. If the subject was "you" instead, this would exclude everyone else from that thought and be directed only at Mary Cooper. You could replace "we" with "you" here, but in this case they have used "we" to emphasize that everyone is thinking the same thing about Sheldon being in his room.