Because it doesn’t need to—and evolution wouldn’t allow it. During short-term fasting, the body has ample fat stores to burn and hormonal changes (especially elevated growth hormone) actively protect muscle tissue. Burning muscle for fuel would be counterproductive: you need muscles to hunt and gather more food. The body preferentially burns fat and preserves lean tissue because that’s what keeps you alive.
This companion explores the evolutionary logic, hormonal protection mechanisms, the research on fasting and muscle retention, when muscle loss does happen, and the myth’s origin. (4 min read)