At some point in time, humanity made a sit down toilet. Once that invention was on the market, something amazing happened… we diverged. Now, it probably didn’t happen immediately, but over time, people decided how to wipe. So I guess, it would have been with the advent of toilet paper that the grand split happened (sit wipers versus standing wipers). You obviously think the other is gross, I’ve had this conversation enough to know that people are extremely aggressive over their wiping position.
But, the point is that in history, someone decided to eradicate the stinky residual excrement while standing up (more than likely the European based society, as you had to move from the toilet to the bidet, in Japan (where Bidet’s became heavily used in the 1900s, they were directly incorporated into the toilet not a secondary fixture you had to move to in the bathroom)). While those humans made a decision to stand up and wipe, another group of society cocked a cheek to the side and scrapped away. So maybe it was a cultural thing, Italians were already standing when they shifted from toilet to bidet and decided this was the easiest way to wipe, or perhaps, it was just an arbitrary decision someone made after taking a dookie.
The outcome is that generation after generation has been ingrained in whatever their first ancestors to use a toilet had done. If the first toilet user in your family line leaned or stood, that decision was passed down through all of your ancestry because you don’t learn wiping your ass from a central power, you learn it from your parents when they potty train you.