Because soap will keep you cleaner. Part of our success as a modern society is hygiene. Soap will keep you cleaner and reduces the chance of illness and help preventing the spreading of diseases. Another reason is after a while you don't smell yourself, might be that you don't have zero odor. Some might have a mild odor, maybe only smelled by others from up close, but repulsing to others. Another reason might be simply why not try a different soap IF he hasn't tried it in the past? Why give up right away?
This kind of works on the assumption that having 0 germs on your body is better, which I don't think necessarily needs to be the case.
Using soap will technically lead you to be closer to "clean" but that doesn't mean not using soap would make you dirty - and if there are genuine benefits to be had with not using soap, why should they be dismissed simply for the fruitless ambition of being "cleaner"?
You mention the smell thing, can I ask, have you tried this yourself? Because I'm thinking you might not have and all of your assumptions are kind of baseless. You're using a lot of theoretical examples rather than real examples.
I've tried it. I go to school with my girlfriend of over 4 years so we spend a lot of time very near each other. I haven't used soap during my showers for over 5 weeks, I believe nearly 6 now and she hasn't had a single complaint. I didn't even tell her about it for the first 4 weeks because I wanted to see genuine reactions and she didn't notice a single difference in scent anywhere.
I also spend a lot of time in packed class rooms and have never gotten an odd glance or anything of the like. There are a lot of people in this thread saying the same.
So you say he should try new soaps because "soap will keep you cleaner" but you don't really give a legitimate reason for "to what end". You mention that it'll reduce your chances of illness and spreading disease but - c'mon, really? I'm not going to pretend not using soap is as effective as using it but I really, really doubt it's to the level where everyone bathing only with water is just some walking disease monger.
And in response to
Another reason might be simply why not try a different soap IF he hasn't tried it in the past? Why give up right away?
Because it's free to not. He found a method that costs him absolutely nothing, saves him time, and has stellar results. Why would he look for an alternative that's more time consuming, costs more money, and has unknown results?