Why didn't Voldemort steal the sword of Gryffindor when he killed Griphook in Harry Potter and the Deathly Hallows? He could have prevented his death.
According to wikia, "The sword vanished from Griphook's hand. " after Voldemort killed him.
Also, the sword "must be taken under conditions of need and valor", so I don't think Voldemort qualifies. Gryphook may be an exeption as he is a gobelin, and may be immune to the enchantements of the sword.
The wiki does mention the sword vanishing from Griphook's hand and also shows a gif of this, but who knows about the canon of this? I don't have a book with me now, but my guess is that even if the sword did not vanish, it would've been too hard for Voldemort to get it. Firstly, he killed Griphook with a curse, so he wasn't necessarily near Griphook. Secondly, the sword can only be wielded by a true Gryffindor, or presumably by a goblin since it is goblin-made. In addition, the sword would probably have disappeared from Voldy so it could later appear to Neville through the Sorting Hat.
There's no mention of Griphook at all after he takes the sword in the book. The wiki says he is one of the goblins who Voldy kills after he finds out about the break-in to Gringotts, but the book doesn't mention that it's him. And if it is, the book doesn't mention the sword there at all. We must assume that Voldy never had any way to access the sword, much less to take it in the right condition (namely, as a Gryffindor and in conditions of need and valor).