Wands are extremely personal items for a wizard, almost as much a part of themselves as any real body part. Without it they are defenseless. Giving up their wand would be like declawing a cat, leaving it defenseless.
Besides, no one at that table trusts Voldy. No one is there out of loyalty, but fear. As Voldemort says to them, not one of them tracks him down after his death at the Potters' hands, no one but Peter Pettigrew.
And Voldy is the type of person that would use you one second and kill you the next.
Anyone voluntarily giving their only line of defense up to a mad man is just as mad as him. Even though Voldy is stronger, each knew that without a wand they go from a handicap to fish in a barrel. With a wand they may have a chance, sans wand it would be a slaughter.
As Slytherins, they all tend to have their own self interests at heart. Each knew it would be best for them to stay quiet until their hands were forced. No need to give up until their bluff is called. A calculated risk based on group actions. There are n wands and Voldy only needs one right?