> The shape of a hammer is more about inertia and leverage than the geometry of your grip, while the shape of a pencil is more about your hand than about the properties of graphite.
While that's illustrative for the real point, its not especially true, is it? Go look at the many shapes of hammer available at a hardware store, "how it is held" is a major factor in the design of the tool. You'll see the same head with 3 or more handle variations because people are not all the same shape.
A pencil isn't designed for the hand that holds it, but for the machines that make them by the million. Tools designed for hands tend to have flanges and bumps to grip and curves to meet the palm: look at the variety of pistol handles, for example.
The handle shape of a hammer is often a matter of marketing rather than utility. The fact that a hammer has a handle rather than being akin to rock without a stick is all about the physics.
Conversely, nearly all the physics-of-graphite differences among pencils are independent of shape...HB, 8B, and 4H all come in the same shape. The same shape that is used for ink.
The closest thing to a pistol handle in pencils might be the oblique nib holder for calligraphy. But that is entirely driven by physics in the interaction between nib shape and expectation of line shape.
A hammer that doesn't fit your hand can do the same job but it will cost you a lot more in effort, pain, and misplaced strikes. One that fits your hand, arm, shoulder, and body will be a tool you can use for hours without fatigue and without inaccuracy.
Imagine typing on a chiclet keyboard 8 hours a day. With EDLIN. Thats what its like to try using a tool that doesn't fit.
While that's illustrative for the real point, its not especially true, is it? Go look at the many shapes of hammer available at a hardware store, "how it is held" is a major factor in the design of the tool. You'll see the same head with 3 or more handle variations because people are not all the same shape.
A pencil isn't designed for the hand that holds it, but for the machines that make them by the million. Tools designed for hands tend to have flanges and bumps to grip and curves to meet the palm: look at the variety of pistol handles, for example.