User blog comment:Majestic Soaring Cloud/Injuries in Bleach/@comment-11051098-20130708144018

Regarding Yamamoto and his arm - Yhwach stated before he incinerated his body that Yamamoto "didn't want to use a human being". So it appears his pride prevented him from asking Orihime to restore his left arm.

I think it was also stated, at least in the anime, sometimes after he used Ittô Kasô, that he viewed the loss of his arm as a "war injury" or something like that, and that he never healed such injuries.