if you think carefully, playing beach volleyball involves leg muscles for you to jump and sprint, back muscle to pull back your upper body in anticipation for a smack, then finally your chest, arms and abs to slam in down. It is still a full body motion/sport.
Think of it, other than chicken legs. Another common term coined for people who trains only chests and arms is gorilla. You should know why.
Sometimes, things are not about whether you care a not. This is just my advice. You can take it, you can don't bother. I don't control you. It is ultimately your own body.
Good luck