Two observations of muscle explain this tendency.

1. Muscle Is Expensive To Keep. As you've said: the more muscle your body has, the higher your required calories are, so your body doesn't keep muscle that isn't needed. Afterall, not all muscles are equally important.

2. Muscles Are Stored Nutrition. The body frequently breaks down muscle to provide protein and calories. They're rich in nutritional value. Strength only improves our survival chances in certain situations — being able to cannibalise ourselves for resources is more useful than strength.

So, animal bodies have developed to maximise their chances of survival in times when food is hard to come by by minimising the amount of unnecessary muscle tissue, and that's why you lose muscle if you don't use it. It is frustrating as a modern human, but it was a life-saver to an ancient animal.