We evolved for survival. Are bodies survived by doing certain things that benefited us. One of the things we used to do to survive was move around a lot. Our system is adapted to be healthy when we move. Even though we don’t move as much now, our body is still adapted for maintaining itself by movement. Think of working harder as “maintenance” for long term use.
For example, our lymph system (immune system) is made of one-way valves that circulate with movement. No movement leads to poor circulation which leads to lower immune response. This is just one example of many parts of us that is adapted for the environment we once lived in for a very long time.