I imagine it was more a practical thing. Making tight pants (e.g. because you’re horseback riding) plus whatever you’ve got going on for underwear in the 18th century probably resulted in some weird bunching in the groin. Speaking as someone with muscular legs, it’s kinda hard to find heavier weight dress pants that don’t make it look like I’m packing way more than what I’ve actually got going on around the zipper area. See also: https://fashionhistory.fitnyc.edu/1797-girodet-belley/