The other commenter makes directly salient points but one thing that can happen is that they expand their speaking engagement market by getting hired for events reaching broader audiences using the lessons they learned in their niche. That's how a relative nobody ends up making five figures per speaking engagement (pulling in six- and seven-figures per is going to be for known celebrities/public figures.) Then there's book and other media deals...

It really messed with my worldview when I learned how some people became rich talking about a thing they only might be actually knowledgeable about, only because they were good enough at talking about it. Or simply because they got lucky with some otherwise random factor (often merely timing of some sort.)

What I REALLY hated discovering were the motivational speaker types making money selling (at the time) tapes and such talking about how to be successful at fucking MLM. These are people who made a bunch of money because they were at or near the top of the pyramid. Just pure, unfettered conning.