@boffbowsh

"Set" is a transitive verb, i.e. you do it to something else, in this case, your feet.

"Step" is not, it is intransitive and doesn't require another verb, you simply do. "I take a step" doesn't require feet to make sense.

"I will not set foot in there" and "I will never take another step in there" have a similar meaning, but are not equal.