Many people have a fascination with cleaning their body parts and looking at the cleaning product to confirm that the dirty stuff is gone. Toilet paper gives you that’s satisfaction.
Unlike hands, you can't visually inspect your butt as you wash it to ensure that it's clean—you just have to trust that it's working. I do see the attraction to being able spot-check with toilet paper to see that things are coming clean.
The point isn't hands vs. butt and visibility. It is whether or not TP actually does the job of cleaning and/or just provides some sort of spot-check, which in reality, isn't cleaning anything.
In this case, our noses are probably a better metric. If you used your fingers to touch your ass after wiping and then smelled them, would you wipe more with dry TP, or would you wash it off another way?
It is odd to me that anyone would argue that TP would do a better job in this case.