Free Training in Exchange for a Cut of Future Earnings: Why does no coach do that?

Strangely enough, success coaches, NLP masters, hypnotists, and other experts with supposedly close-to-magical powers never agree to free training in exchange of a percentage of the clients' future earnings. If their claims were true and just a minority would pay something like 10% of a multi-million income, this should be a great investment. Yes, I've asked, but was always turned down...
I can understand that novice coaches can't wait that long for their cash flow, but established names should be able to do so.