Resurrection of the Dead is the foundation of the entire Torah and the root of the mitzvos

 Rabbeinu Bachya (Kad HaKemach Pesach 3) It is known that the Resurrection of the Dead is the foundation of the entire Torah and the root of the mitzvos. That time is the joy of the soul together with the body as it is known that the body is an instrument of the soul because it shows its activity and it enables it through doing mitzvos to merit Olam Habah Therefore they receive rewards together.. Because only together can mitzvos be done and therefore G-d gives reward to both together.