I think that is a flawed question--as it frames things poorly.
I see no evidence that it "the way things are" at all.
Plus I see no evidence that MMA are willing to accpet "challenges" in general--some certainly do--but certainly not "all" or even "most."
Heck, I bet you can't prove "many."
Its a sure bet the "pro's" WON'T--Chuck Liddel would just laugh at me if I "challanged" him.
He has waaaaayyyy to much to lose if he accedently breaks his hand on my face or slips in my blood and hurts his knee.
Plus, there is, as I see it, a huge difference in outlook being expressed here.
I don't train to get "challanged" to go mano-a-mano with some idiot who stumbles into my practice and wants to "go."
I train to defend myself vs a violent attack.
When I can't get away and can't get my gun out or can't get to a weapon useable object.
If its not worth putting a person in the hospital over--killing someone over then it simply is not worth "really" fighting about.
Opinions will vary, but that is my view.
Besides anything less than "really" fghting is going to come down to the rules.
And whomever trains under those rules will have a HUGE advantage.
Can you imagine me pulling a knife during a MMA match??
Hey, under some peoples "rules" the use of weapons is a "given."
To paraphrase a guy I know "I have a good, job, a pretty wife, couple of kids, way too much to risk getting seriously hurt in some stupid match."
So when you ask "why don't TMA accept challanges??" a MUCH BETTER question is:
"Why should we?"