The whole point of a fetish is that it's weird and unexplainable.
Lots of men have fetishes about women's underwear (or other clothes) that have been worn by a woman and will specifically seek it out on eBay - and pay good money for it. There's even an underground code, so I hear, whereby women who have these men as customers for their wares describe them as 'cleaned to eBay standards' - which means worn and not cleaned afterwards.
They must save a fortune and loads of time on their washing - if they can just get job lots delivered, wear a pair for one day, then package it up and send it off to a perv gentleman, and move on to the next new pair the next day!
Still, some men have fetishes involving wee and poo and equally grim stuff... so I guess, by comparison, getting off on 'just a foot' is far preferable!
It's also less risky for women who use OF and specialise in only 'showcasing' their feet, as they get to stay anonymous and thus safer. Not that it's not grim, but much safer than having your face all over those sites.
I've no idea how these men end up with such bizarre and/or disgusting fetishes - but they clearly do exist in large numbers.