https://www.audacy.com/kluv/latest/biologist-explains-why-some-people-have-foot-fetishes
Well according to biologist Forrest Valkai, it's all because of the brain.
Valkai explained that foot fetishes occur due to the part of the brain called the somatosensory cortex, which is the area that makes "you feel things."
Apparently, this part of the brain is right next to the part of the brain that control your feet and toes. He explains, "It's so easy for these wires to get crossed because they're literally bumping up against each other. That's why foot fetishes are the most common fetishes in the world."