>>149302985
The dynamic is different.
When a manga has a different artist and writer, that duo is more like a married couple. The two will be working very closely together, often even in the same studio everyday, and the authorship is shared between them with them both working together to produce the story

In comics, the writer produces the entire story and then hires an artist to draw it, sometimes it's not even the writer who hires them but instead an editor, and it's common for them to not even communicate with eachother outside of notes written on the page.
The Marvel Method operated like this.

I often bring up Scott Pilgrim to illustrate this point
Ask yourself, if you were an artist, what information would you require to draw a page of Scott Pilgrim?
The answer is obvious, you would need a complete, deep and intimate understanding of the story and all the nuances of its characters. Otherwise you wouldn't be able to create a convincing expression on the faces in panels that on the surface appear unimportant, but in the grander scheme are extremely important to each characters' life