I think the "competitive" piece is more important than the "can't verify" piece. I'd add academics, politicians, and executives to that list.
A lot of professions are structured like pyramid schemes, with partner/tenure/presidency/CEO at the top, and crap at the bottom.
On the flip side, my experience is most tradespeople are quite scrupulous, not because I can verify the work, but due to lack of competition. If my handyman takes 30% longer to work, he perhaps makes 30% less money, but life goes on. If an academic misses tenure, or a politician gets knocked out of office, careers end.
