The King of Gondor is the only one who can command the ghosts. But they have free will. They’re options are “follow the King of Gondor like they should have 3000 years ago” or “just keep being ghosts.” But they’re cowards. They’re ghosts because they’re cowards and ran away 3000 years ago. Of course their initial response to “you wanna follow me into battle?” is gonna be no. Especially up against Sauron, who I don’t know with 100 percent certainty can’t hurt a ghost. He’s kind of a necromancer, kind of a Lich. He might know how to hurt ghosts, so those ghosts might not be an invulnerable as we think.