Quick answer: Your cat bites your feet because moving feet look exactly like prey — the movement triggers hunting instinct, especially in young or under-stimulated cats.
Feet and ankles moving under covers or across a floor simulate the movement of small animals. Kittens and young cats with high prey drive are most likely to ambush feet. It's hunting play directed at the most available moving target — not aggression, and not personal.
Indoor cats without enough active play redirect hunting energy onto household movement. Two 10-15 minute daily play sessions with a wand toy or electronic motion toy significantly reduce ambush behavior by giving the prey drive a proper outlet.
Never use hands or feet as play objects, even when it seems harmless as a kitten — it teaches that human limbs are appropriate targets. When biting happens, disengage calmly without dramatic reaction, then redirect immediately to a toy. Don't punish — redirect.
Foot biting is a prey drive issue, not aggression. More interactive daily play and never using feet as toys is the complete solution for most cats.