The human brain is weird. Genitals and to a lesser extent, breasts and hips are the only things we should care about, but all the little parts of the body carry their own subconscious value. It’s not my thing, but having a foot fetish comes from the same part of the brain that likes any other part of the body that isn’t strictly related to reproduction. Legs, ass, shoulders, neck, hair and skin, etc. (and this is really just the cis male perspective, tip of the ice berg)