"Scientifically it's because feet coincidentally are not often shown unless wearing revealing clothing/none at all, and in certain people, the brain somehow associates them with other parts that are
also not often shown.
And unfortunately many people wear footwear more due to it being a relatively common fetish, and this can further reinforce it in an individual and society as a whole.
It's not that bad, there's plenty of worse ones and trust me, i know them quite well.
(For clarification, i do not have a foot fetish.)" -π€