Why don't they just use another plane? For example. I am sitting in Dulles right now. My flight to RDU is delayed by four hours. We are the last flight out. The entire crew is here ready to go as soon as the plane gets here. I am sure there is some other A319 sitting at a gate that's not going anywhere tonight. Why can't we just use that plane? And then whichever plane we use, uses our plane whenever it gets here in the morning?

Maybe I am just thinking about it to hard. It sounds simple but I am sure there are many factors at hand. Just curious. But it seems so simple to me. Please explain.