There's a tv credit card commercial which asks the burning question "what's in YOUR wallet?" It's a fair question for martial arts practitioners, too. Extraordinary claims demand extraordinary demonstrations validating said claims. Invariably, fakes and phonies claim to have skills which they never personally demonstrate and never teach to their students, because they don't actually possess them.
Those who enjoy fame and fortune from their teaching, regardless of whatever that may be, usually have very little, if anything at all, to gain in accepting challenges from others who have less to lose if they are defeated. It seems reasonable, therefore, that most famous teachers would be reluctant to cross hands with everyone who wants to test them.
They may need a better reason to risk engaging in serious fighting than simply satisfying someone's curiosity about their real skill level. It may be a professional payday for some people (i.e. pro fighter, pro bodyguard escort, pro security personnel, etc), or only to defend themselves, their loved ones, or their property for others. Just saying. What do you have to lose?