You're asking for a rational explanation for something that is inherently irrational.
I can't explain to you why I feel repulsed by the sight of bare feet, because I own a pair myself, so I shouldn't be in any way squeamish about them, but I am.
I don't even like looking at my own feet. They're not really any different to anyone else's, but feet themselves are an inherently displeasing thing. They're ugly, a stupid shape, some of them are hairy, which is in itself utterly repulsive, then you have people with just outright terrible feet covered in dead, yellowing skin, broken, fugally-infected nails. Folk whose feet utterly stink and seem to take a bizarre pride in the fact it makes everyone else's stomach churn...
I'll stop now because it's making me feel bilious just typing it out. To be blunt, I don't want to look at someone's feet any more that I want them to bend over so I can closely inspect their bumhole, and I doubt many people would think that's an odd thing to be repelled by, but they are both just as repulsive to me.