Because aiming a stream of water at your anus and having it wash feces down your legs (all over your underwear and pants, in and on your shoes) is sanitary. ?
I’m not familiar with the specifics of this equipment. I do know that our present systems aren’t nearly as sanitary as people like to think.
Cleaning the anal region with soap and water, then rinsing is IMO by definition more sanitary than wiping the fecal material off with paper.
You don’t do it standing up, which your comment suggests.