I think maybe it's effort? Like no matter how bad an author is at the beginning, they will improve if they practice. They have to practice correctly, though. That means trying new things and actively working on your flaws.
Take El Goonish Shive. It's got quite good art now- recent page for example:
but it started out in 2002 looking like this:
Yet even in the earlier comics, we see the author trying to improve. He's drawing backgrounds, action scenes, expressions, and he puts effort into his comic.
I've seen other long running comics that start off similarly, but don't improve.
Like Rain, a comic that I think actually has pretty decent writing. But I think the author cares a lot more about her story than her art.
Here it is in 2010:
And here it is almost a decade later, in 2019:
It looks almost exactly the same to me.
I think the author of Rain tends to default to the same angles and expression a lot, and is happy with the way her art is so doesn't try anything new.
Reading the author's notes, the author of El Goonish Shive is always varying his eye shapes and proportions, and trying out new coloring techniques and ways to draw action. He's also crazy meticulous in making character sheets, and keeping track of how places are laid out and how tall characters are relative to one another.
Meanwhile Rain's author's notes are always about the story- I think the only time she's commented on her art is to say what type of brush she used.
So anyways, in conclusion it's only two data points but I thought it was interesting how different these two artists' journeys was.
Maybe someone else can pick up on differences in these two comics, or other comics, that I didn't.
It'd be cool to hear alternative explanations
