Gaining muscle is a lot more than just breaking down the tissue and then waiting for it to rebuild bigger and stronger. Weight lifting generates a huge hormonal response that completely changes your body’s metabolism, not just in the one area that you worked.
The reason that most people are unable to build only one muscle group is that neglecting the others means that 90% of your body is not contributing to you overall metabolism.
You go to the gym monday and do chest (which is also obviously working your triceps and delts too) and leave the gym with elevated levels of HGH, much lower levels of insulin, much higher cardiac output, increased respiration and oxygen delivery to your tissues. Then you go home and pound down a huge meal of carbohydrates and proteins.
Do you think these effects are limited to your chest?? Of course not! Obviously your chest is going to need more of those nutrients to repair, but every organ in your body feels these increases. The difference is that if you are not creating a nutrient deficit prior to this loading, most of it will not be absorbed and will just turn to fat.
There’s your long answer to a short question.