Why do people have training shoe fetishes?

Who knows? Humans can develop fetishes for almost anything: leather, silk, shiny red vinyl boots -- if you can think of it, someone probably has a fetish for it. (A fetish for training shoes is not all that unusual, by the way.)

Our ability to develop fetishes is probably one of nature's ways to increase the chances of sexual intercourse: two people who wouldn't normally get together might be attracted by their common fetish for training shoes, and that could increase the chances that their genes would be passed to another generation.