I think there's a lot of ethical reasons not to use AI. A lot of images generated by it, are stolen from across the internet, and sometimes it generates art that is very close to other artists art.
But if you're talking purely utilitarian reasons, I think it's just that there's no reason to use it.
If we are producing a comic that requires a certain scene, then having full control of how that scene is depicted, makes the most sense. Each stroke of the brush is intentional by the artist, in order to build a scene with a consistent vision.
The character expression, the world shown in the background, the body language of the character. All of these combine together to tell a full story.
Illustrations generated by AI might at first glance look good, but they lack the intentional quality of something created by an artist.
The body language, expression, lines, everything will only be an amalgamation of what the AI model was trained on. There is no intention behind it. That's what people mean when they say AI generated art feels like it lacks soul.
You might as well ask me why I don't produce a comic using entirely stock photos. Sure, technically I could, and perhaps there are enough stock photos to make a somewhat coherent story. But if I took a camera, hired a few models, and then went out and took my own photos, that are made specifically for my vision of a photo-comic... do you not feel it would be better?
It would be intentional. It would be made for the purpose of the story I intended to tell. It would be consistent.
I view this intentional consistency as worth more than each individual pictures superficial "quality".
This is why I do not believe that AI can ever truly replace human artists.
You might then ask me, why I don't use it as a tool?
Well, what would that tool be used for?
Anything AI can generate, a human can do better, with more intention and consistency. With more soul.
So what purpose would using AI as a tool, even partially, bring?