Eh, if I can offer some insight to this, from a character designer AND animator's POV...
A lack of nipples is just straight up weird, I mean, yeah . . . but the hair thing is sorta justified.
First off, in comics or any kind of art form, it can become incredibly tedious and pointless to add such details for no reason other than "realistic accuracy". If you watch animated films, you'll notice half the time human characters aren't even drawn with fingernails (Ursula from the Little Mermaid is one of the few from the 2D animated Disney films! this wasn't as much of a problem in 3D films for obvious reasons lol) We don't have to worry about it being a "budgeting" thing as much though, seeing as how we're all devoting our own time (and money) to our personal work, but that brings me to Point #2...
Unlike nipples, hair can be a rather defining feature. Yes, every character has nipples, but not every character has thick arm hair or leg hair or chest hair (some guys can't even grow facial hair). There are certain details that, though seem small, can end up adding to a character's, well, character (or even detract from it).
For example, if I see a character that has short black hair, thin blue eyes, and a lean build? Those characters tend to be very mysterious, quiet, solemn. These characters also usually have no body hair whatsoever.
However, then if I see a character with facial hair, arm hair, chest hair, the like? My brain is automatically going to think "Oh, this character looks really rough and tumble! He looks like a guy who doesn't care about consequences and just goes head first into danger!"
That's why Hercules doesn't have body hair but Gaston has shitloads (besides the fact that they only had to show his chest hair for like, 4 frames - if they had him shirtless more during the film, they would have gotten rid of the hair because that would have cost WAY too much to animate; that's why they didn't "realistically" give jungle man Tarzan any body hair besides his dreads). Quasimodo however DOES have arm hair, and they actually maintain it throughout the film because it adds to his character - from his back hump issue to his isolation within the clock tower, everything that's a part of his physical body is just meant to scream "weird outsider". Even his hands are drawn in a deformed way, with very clubbed fingers and thumbs; this is an example of Disney really going out of their way to create a realistic character in a realistic time period with realistic culture and history and consequences (hell, the movie's plot is about Romanian genocide committed on the basis of religious paranoia...)
Yes, it's "just body hair" but it's one of those things that can really change how a character is portrayed and how their persona clicks with readers.
I would totally draw Coach Ruin with body hair. He would have like, forests of chest hair lol (but I haven't had a reason to draw him without a shirt lol)
But I'd be damned if I drew even a single hair on Mitsuhiro's body that wasn't on his head or face. It just wouldn't work for him. Besides the fact that he's the second leading protagonist with the most page-time alongside Uzuki and Saichi - unlike Coach Ruin who only shows up every few chapters, going to the effort of drawing arm hair and chest hair and such on him every single frame would get tiring and just wouldn't make much of a difference to the structure of a page or the plot in the grand scheme of things.
If it's any consolation though, no matter who I'm drawing shirtless, they're always going to have nipples 
