From my Spaniard point of view, there's three things that I find fascinatingly weird about Americans: Guns, boys' circumcision and shower loofahs. Why do you need a loofah to wash your body? Do you use a loofah too when you wash your hands?
Welsh, not American, but I find that the loofah type puffy things get me much cleaner than when I just use my hands. They let me scrub properly, rather than just rubbing soap onto my skin.
Loofahs are not as common as you imply. Like, they're available, and some people use them. But I think it's more likely someone uses a washcloth or just their hands.
As for why? I'm generally a "use my hands" man. But I do use a wash cloth if I'm particularly dirty for some reason. It gives a bit of exfoliating action that scrapes off more grime than just my hands.
So it’s better to stick your loofah in you ass and then put it all over your body?
You can wash your ass with your hands and wash your hands right there in the shower if you think they are so dirty, which they won’t be because they are full of soap and water is running over them