From what I understand, the Berlin museum’s excuse for refusing the loan to Egypt was that the bust was too fragile to be moved. Yet the same museum managed to move it for several hours from the glass case to pose it for photos on top of a bronze body torso. The bust was also recently CT scanned, but I haven’t been able to find out if it was moved out of the case to do that. The museum seems willing to move it for their own purposes, but not for it to be on loan. But frankly, the fragile excuse is just that...an excuse. They’re simply afraid that if Egypt got its hands on the bust, they’d never get it back.
Moving it carefully by hand a few feet or into another room is quite different from moving it to and from a country hundreds of miles away, so I don’t find that to be merely an excuse. There was relief throughout the art and ancient history communities when the straw-lined box containing the bust reemerge in postwar Berlin (and unlike the gold of Troy didn’t vanish into Soviet booty archives).
But if it were made out of obsidian, the Germans could just tell Egypt, “forget it — you won’t return it” and would be right about that.