Umar is pretty controversial, but most people would probably consider him the surgeon with the most BHT experience.
Just remember, for every 1 HT that ends up like that, there may be 5 that fails. We don't know.
Also, the density is not that great for 20000 grafts (yes, 20k) which goes to show that the growth rate of BH's is significantly less than regular hair.
On top of that, 20000 BHT would be rediculously expensive. And finally, not everyone has the body hair for it.