Muscles are incredibly inefficient.
That's why people that work 12 hour days doing manual labor were "skinny strong".instead of jacked.
To get huge muscles, we need to trick our bodies into thinking we randomly have to move heavy weights every once and a while.
Part of that is slowly increasing the weight so they muscle rebuilds even bigger otherwise it would just stay the same. If you stop, your body stops wasting the energy for those huge muscles and they get smaller.