He might be to prideful. His dad never spoke of that scar because it was his life's shame. How do you think the raikage would feel about losing an arm? Second theory. He might not know anyone that knows earth grudge fear to sew an arm back on. Third theory- he might need that specific arm (chakra network and what not), but its ash now. If he got froggy and really missed his arm he could try learning oral rebirth I suppose, but I think deep down the villages know there will be more wars in the future and they don't want to share such useful techniques.