The percents are very different. For example in bodybuilding one normally 'bulks' while working out, because it maximizes muscle gain. But then naturally this needs to be paired with cutting, unless you're a Greek Grizzly, but the total muscle loss is relatively negligible, especially when maintaining a proper high protein diet.

At 40% muscle loss you're getting awful close to losing weight while increasing your body fat percent!

But of course you're right that diet+exercise is key but for those maintaining such, they wouldn't end up on these drugs to start with.

For example in bodybuilding one normally 'bulks' while working out, because it maximizes muscle gain. But then naturally this needs to be paired with cutting

This comes from professional bodybuilding, where people are using steroids, along with various, uh, interesting chemicals on the cut[1]. It has almost no benefit to (real) natural bodybuilders. It's closely tied to cycles of steroids.

[1] Ephedrine, Albuterol, Clenbuterol (literally only approved for horses in the US), DNP, and probably more that I haven't heard of. Here's an NIH article on the dangers of DNP, to put it in perspective: https://pmc.ncbi.nlm.nih.gov/articles/PMC3550200/