This is exactly right. Scissors depend on you to exert sideways pressure on the blades. It should be possible to build scissors with e.g. a spring at the joint

This is exactly right. Scissors depend on you to exert sideways pressure on the blades. It should be possible to build scissors with e.g. a spring at the joint that exerts the proper sideways force automatically. This is how paper cutters work. (A square surface with a blade on the side that swings down.)