It's not about losing the building muscle. It's about losing the motivation to do so in tech companies. After many years of being IC, things can get boring. You keep doing the same things, even if you move companies and technologies change. And you always do it in a sub-optimal environment - things like working with tooling you don't like, under managers or with co-workers you don't really like, on projects you're not super excited about, etc. So moving to management is almost inevitable. Not to mention there are other reasons such as better salary and more visibility to change things. Also, people need change in their life. Moving to management is one way to change things. Some people go back to being IC, but most don't.