As a developer you work hard to solve complex problems in logic and you invest time and energy to learn a lot of different tools, languages, frameworks and all their quirks and gotchas. You spend years to perfect your craft, learn the codebase and improving the code quality. If you work for a company that values your skills, you will get a budget and time for training and improving your skills as well. Which is important because technologies are always changing, evolving and being updated and upgraded. Some years later you will be fluent in computer language and get rewarded for your hard work with a new title and promotion of dev manager.
Suddenly your experience and knowledge on how to make machines do what you want needs to be transferred to making people do what the company wants. And that’s where it goes wrong. Firstly all your up to date knowledge of the codebase, will wither really fast by no longer working in it. Your language knowledge will soon be outdated because you are no longer keeping up with it for work, and soon all those years invested to get to the role of senior developer will be for nothing. Secondly, all that time spend improving your technical skills, thinking logically, practicing math and order, are the opposite of managing people. People are not logical creatures, you need a certain likeability, leadership skills and soft skills. You spend years learning these skills, practicing them and training them, suddenly you need to switch around your whole skillset.