And by being better at it than anyone else, they'll naturally become professionals in that area
Complete non-sequitur. There is much more to being a professional than the ability to sing or play.
Not a complete non-sequitur. Let's say they have the *opportunity* to become professionals in that area, then. If you're not better at it than other people though, lack of ability rules out that profession for you.
Giving up a secure, if dull, job for life on the road and an uncertain future when you've got obligations to family and social ties where you live is not a light decision either now or 200 years go. Is it inconceivable that someone could be good at something purely for the love of it without getting paid?
Not at all - I'm one of them. :-) But because I'm not putting the time in on music, I accept that I'm only doing it for fun. I don't expect to be an aspirational figure for anyone to look up to and say "I want to be able to play like him", the way you would with people like Michael McGoldrick, Bob Brozman or Doc Watson. Nor do I expect to contribute significantly to human knowledge or experience in what I do musically.
And being "good at something" requires practise. If your day-job is music, then you might well practise 7 hours a day (when touring permits). The professional who can put in the practise time is almost always going to be better than the part-time amateur who can't. And the professional can't do that unless they have another source of income, which typically means getting paid for playing. (Or these days, retiring and having a secure pension.)
By the way, let's remember about the "uncertain future" that generally choosing music as a profession is almost always for young adults whose future is *inherently* uncertain.