To this day, I still don't know why people are into this
For most of western history, there has been a cultural expectation that sexual and romantic relationships must always revolve around a Dominant Man and a Submissive Woman, and even as more and more communities try to resist or defy that patriarchal history, the expectation that sex has a Dominant party and a Submissive party has proven harder to shake - radical reform of political institutions is one thing, but some things are more stubborn! So a lot of circles, wanting to defy the dominant man/submissive woman expectation but having a hard time fully breaking from the dom/sub structure, simply flip the poles around, with a dominant woman and a submissive man.
But the thing you have to understand about fetishism is that it compulsively escalates everything. So once certain people decide that It's Nice when A Girl Calls The Shots, they start wanting to see that girl be more and more dominant, and then even more dominant than that, and then even more dominant than that, and that road can only ever end in one place.
I think it's more that men are trying to prove themselves by showing how much abuse they can take. Might also be that women can prove how soft a touch they have by using their feet. Then there's the trust building aspect.
To this day, I still don't know why people are into this
Personally, I think it essentially boils down to a (bizarre but unique) way of expressing love in a submissive fashion. Having someone step on you (besides giving sexual gratification) usually means you're giving up control to them. You're basically giving entrusting yourself to them, fully trusting that they have everything under control which is a pretty big commitment to make. One can perceive being stepped on as being very intimate akin to a hug or kiss as you wouldn't do so with any random person, only those you love. For others (mainly masochistic people), them hurting you can be seen as an expression of their love for yourself. Conversely, giving someone dominance over yourself is a massive show of your trust and faith in them, which can be seen as an (strange but intimate) expression of your love for them.
So, basically getting stepped on just shows how much you love someone or someone love you (at least to me).