We've seen in books how hard a wizard can hold on to their wand (and other objects) even when it's being pulled out their hand with some force. Which would make it very hard for it to be just yanked out of the hand. Add to that the fact that whilst he still holds the wand, Voldemort can cast a spell to disable Palpatine and interrupt him mid flow.
Indeed, Voldemort could then Accio Palpie's Lightsaber and take away his main hand-to-hand weapon.
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