I think I read somewhere that having a foot fetish is genuinely being built different. As in, "some wires in the brain got crossed", built different.Better than me. I'm so boring that I don't even understand why people like feet.
Something like a part of the brain that processes pleasure, stimulation, or something along those lines is really close to a part that processes the sensation someone gets from seeing feet, and that's why it's so common.
Don't know if it's actually true at all, but it's funny if it is.