You do what the person with the gun says, because you believe they'll shoot if you don't. If you believe that they will shoot and kill you regardless, following their orders is (at best) going to give you a few more agonizing minutes to live. The threat becomes meaningless.
Don't try to overpower the hijackers? You die. Try to overpower the hijackers and fail? You die. Try to overpower the hijackers and succeed? You live. It only takes one person to do the math and realize they are basically in a no-loss scenario.
Don't try to overpower the hijackers? You die. Try to overpower the hijackers and fail? You die. Try to overpower the hijackers and succeed? You live. It only takes one person to do the math and realize they are basically in a no-loss scenario.