It's not clear to me that independent tutors are generally getting rich. But there clearly are activities that benefit from individual training. I imagine music is one of those. But that mostly probably falls into the luxury goods category.

"Getting rich" is a substantial movement of the goalposts. The point is that people are spending money outside the trade paperback range on education, either for themselves or for their children (I've seen quite a wide distribution while in the waiting room), that has nothing to do with income.