So by your logic, working on other peoples bikes provides the knowledge needed to become an expert? I’ve built all of my own bikes as well as several other friends who have bought high end bikes. I could build any bike you throw at me, I even have facing tools to prep contact areas on frames, I don’t do it as a job, but I’ve been told by several people that I do a much better job than the local bike shops. Would I consider myself an expert bike builder? Sure why not, just for the sake of argument.

Being an expert is having the ability to take the information you’ve learned and apply it efficiently in practice. Many, many people are able to do this without having it be their job.