Peter couldn't leave the Order, of course, but at least he had a built-in excuse. I'm not as strong as you guys, I can't just leave, it wouldn't feel right...
"Yeah, whenever, Remus," Peter agreed, and glanced at James with a disapproving little frown. "Isn't this hard enough on Moony without you guys wanting to punch them in the face?" he added in a stage whisper that Remus could, of course, hear, since he was right there.