Joseann
I guess the thing is, that the contestant doesn't really know how they are going to react in a challenging physical situation. So they sign up thinking they can cope with anything thrown at them.
And dance training is tough and relentless.
It's interesting that the only sportsperson who has complained (as far as I'm aware) is Will Bayley, and again, I'm not belittling his achievements in any way, but I would hazard a guess that training for table tennis is quite possibly one of the less robust training regimes.