It's not "just skin". It's skin on the outside with a thin, sensitive muscle sheath underneath and the same exquisitely sensitive mucosa as the inner edges of your lips on the inside. Right around the edge of the foreskin is a ridged band of tissue which attaches at the frenulum to the underside of the glans. (Together, that band and the frenulum are considered the primary erogenous zone in males - the head of the penis is secondary and much less sensitive.) Not to mention tens of thousands of nerve endings and specialized immune cells.