As an artist, I would opine that repeatedly having to draw the same image sometimes results in simplification.
A great example, would be Mickey Mouse. Especially in the early days of animation, a “cell” was drawn, for each frame of a cartoon.
If Mickey had 4 fingers, it would have been more time consuming to produce cartoons.
The Simpsons filled suit, SouthPark is done with a animation computer with simple images.
A clean shaved man, would be easier to draw, without any facial hair.
We see simplification often, like capes. Another thing, that isn’t a coincidence is that capes can cover a large area.
Sunglasses, or a mask, is easier than detailing eyes.
Hats, Headwear, or a braided/simpler hairstyle also can help.
These things are done for mainly one of two reasons;
The artist has trouble with certain things, like hands, or eyes, so they are covered or out of frame.
And like I said, it’s less time consuming, especially in the animation world. Which is practically dead now. Replaced by CGI. The images typically used in CGI, use some of the same tricks, to cover or cut corners, but can really add lots of detail now.
But think about it.
Find an image of Batman.
A nice clean one, from like an old cartoon.
Just the bust, will be needed to “illustrate” my point.
Once you’ve found the image, draw it 9 times, about the same, on a sheet of paper.
Do the same, with a different character, like Wolverine with the 5 o’clock shadow and sideburns. Again, just the bust.
Draw his likeness 9 times in another sheet of paper.
You will notice early on, that you are still poking in facial hair on your first image, when if it were Batman like before, you may be on the 3rd or 4th image.
Homer Simpson is an example, inside an example. As his facial hair (always 5 o’clock shadow,) is represented by just a solid color, with no detail.
Sane with Bart and Lisa’s hair.
Simplification, for production sake, is a factor.
Plus. Earlier heroes, only masked their eyes.
So their identity would be in jeopardy if they have unique facial hair.
I have a big beard. If I were going to rob a bank, I would dam sure cover my beard somehow.
It’s worth noting, that most characters did not have body hair, either.
When Seth McFarlane drew arm hair, he did just simple lines on his character’s forearms, and did address facial hair.