The body doesn't jump straight from carbs to breaking down your muscles. There's also fat, and ketosis is kind of central to the keto diet.

As far as I can tell, the jury's still out on exactly the effects of prolonged ketosis on muscle gain and loss. Although I will agree with you that if you're in shape and your goal is to build muscle, you will definitely want carbs in your diet. But I think your claim that low carb diets always lead to notable muscle loss is disputed by some studies.

Here's a pretty decent article summarizing what we know about the relationship: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6724590/

But this topic is concerned more with exercise for the purpose of mood regulation/energy levels, and different people may prefer different "secondary benefits" such as losing weight or gaining muscle mass.