The problem is as simple as... time. It's not about who's the most important, it's about who spend the most time on the project.
I'm mainly a writer, who turned to drawing because I don't have a good language anymore (lost a lot of my native language and never got to a great level in English).
To be entirely honest, I was never a professional fiction writer, but I wrote non-fiction professionally and was considered a apt writer for that.
In a comic setting, I can imagine cases where I would spend as much time in writing than in illustrating; and would be okay to share 50/50 with a writer if I needed one.
For eg. If the writer needs to do a tremendous amount of research (weeks or months before starting project; the writer needs to learn about a whole new field), and provide the artist with a story already in the form of a storyboard (or at least very easy to convert into one). Or, the text is not that demanding, but the art is super simple, like some slice of life comics. I could even imagine under that last category, a case where the writer would spend significantly more time than the artist, and should be paid in consequence.
But these are specific and rare cases. It looks like the common situation is that for a creator who does both the art and the writing, at a SIMILAR LEVEL of quality, the writing is a smaller portion (often significantly so) of the total time spent on the comic.