1. Gene and tissue problems
Animals and humans have completely different genes and tissues. When humans are hungry or stop using muscles, some molecules in the body will have a chain reaction. They change chromatin and slowly destroy your original muscles. For example, you go to the gym for a long time and exercise your muscles. But if you don't go to the gym for a month, your muscles will be much less than before. Animals are not like this. When they are inactive or hungry, their genes and enzymes will not break down their muscles actively, which makes their muscles last longer.
2. It's useless and unnecessary
The muscles of the body will gradually break down because the body feels that you don't need muscles anymore. If you don't use muscles, the body will automatically understand, and then muscles will begin to degenerate. Some animals are not like this. Even if they hibernate and don't use muscles for a long time, the body won't acquiesce that they don't need muscles. Therefore, the muscles are still in their original state and will not slowly degenerate. For animals, this is also a better place than humans. Some things evolve automatically, so you don't have to worry about changes. Humans can't do it. They have evolved into this state, and you can't help it.
3. Ketosis status
Ketosis is a magical thing. For example, when people are hungry, they consume sugar in their muscles. Animals are different. Even if they are hungry, the first thing is to consume the body's fat reserves, and the muscles will not be affected.
Finally, all animals (including reptiles) may have muscle atrophy due to illness or lack of use, but the crawling body thinks that muscle is necessary and uses fat as energy.