Most hands have blemishes such as scrapes, scars, spots, or discolorations which would stand out in a close up and detract from the product.
For print media hand models, a necessary trait is the ability to pose in a relaxed, graceful fashion.
Professional hand models avoid what photographers call "the claw," the rigid grip that novices often use before the camera.
Many food commercials have hand models pouring gravy, dipping a French fry or cutting meat.
Here perfect hands are not as important as the timing and patience required to "hit one's mark."