Why do men like to lick women's feet?

Because female feet (if taken good care for) smell like female pheromones which is very attractive smell for men. Feet have millions sweat glands each of them produces sweat laced with pheromones. It is (almost) odorless; the "feet" smell is actually produced by bacteria growing in the wet shoe environment. Clean feet do not smell.

On the other hand feet have millions of nerve receptors and for most women are erogenous zone. Some women like when a man gently bites soles, some like when a man slides his tongue between her toes.