In order to successfully grasp something, you must immobilize it against the force of gravity. Friction is the key ingredient here.
This is Coloumb’s Friction Law - which shows that the more normal force the higher the force required to overcome friction.
Also, if you have a small point of contact and the net force is at an angle the object will slip.
Most grasping requires multiple contacts - and in practice deformation is helpful since it distributes the force across the object.
Suction is another great option instead of pinching and simplifies the control aspect of grasping. However, it is limited by the force that can be generated by the vacuum and cannot pickup objects without a large flat surface.