One of my nephews has a metal rod in his left femur because he was doing this ! The accident that did that when he was in his late teens was not the only one he's had and he still says he is most at peace when he is out riding. His Dad rode with HIS Dad and so my nephew and his Dad rode too. Passing it on I suppose.
It all seems crazy to me, with the risks. Good thing everyone doesn't see it like I do, or there would be no one for me to watch ! Lots of things have a little risk, but some obviously more than others.
There is no accounting for what you find you love deeply.
There's inherant risk and danger in freestyle Motocross and when it goes wrong it goes really wrong as your nephew found out. Humans have a way of forging ahead nonetheless usually calculating the risk, but sometimes not, and that's what's progressed us to where we are I guess.
Those people who choose to do this sport love it, it's to their core and one has to applaud that passion I think; even when they gat badly injured they come back for more.
Forging ahead .... if you survive too !
(When he was much younger and an early bike. :) )
It starts small and then, they go big. It's all that effort in between that makes them good At it and able to do those things reasonably safely, or as safely as possible.