It seems to me that compared to other animals like e.g. gorillas, the way our body reacts to exercise or inactivity is different. We need to do quite a bit of hard physical exercise to build up muscle mass and keep all that muscle mass. Gorillas, in contrast, seem to be able to eat and sleep all day long and still be extremely strong.
Of course, they will still get some exercise by moving around, climbing trees, but I don't think gorillas are in the habit of voluntarily pushing themselves to their limits compared to how people in the gym are pushing themselves to their limits. If they climb a tree, then that is exercise, but it's presumably much farther within the limit of their muscle capacity then when we do pull-ups.
So, is there something fundamentally different about human physiology that predisposes us to become couch potatoes much faster than other animals?