I agree, it can be gross and weird to sexualize feet. That's true of sex in general or anything else that's sexulized. We tend to ignore all that gross stuff in favor of, in the moment sexual arousal. Though if ones disgust for something is strong enough, it will block them from the arousal part and all they'll see and feel is the gross part. Which I think is a bias, but you can be the judge of of that for yourself.
The understanding part is... well I suspect you won't want to.
The problem is it can't be explained logically because it's an emotional thing, logic doesn't work here. Short of experiencing it yourself, that leaves self reflection and I mean the serious introspection kind. Why self reflection? Cause the best way to understand others is to understand yourself. Which is to say, ask not why others have a foot fetish, but why you don't. As odd as that may sound, psychology works in weird inverse ways.
So you can red pill yourself, awaken and see your true self and understand. Or just take the blue pill and go play lovelycraft. lol.