To be honest, writing is already a very undervalued profession. As an artist you can make media and illustrations that are separate and for sale. You can go to comic cons, you can promote your comics with a single drawing and art always draws people in. Even on this site, a bad comic goes further than a good novel.
To say that then a writer should make even less on a project than the artist I feel just adds to that narrative that writing is not worth much.
I personally think it is because as an artist it's much easier to show what you have done with your time. You draft a comic or a design and sketch, and these things all show up on paper by definition. The only thing an artist ever sees is a storyboard. And most of you from what I have read are artists and artist-writers that make their own storyboards. But that is not the same as a dedicated writer for a project, especially not if they are good.
A dedicated project writer is your researcher, your marketer and provides you both with foundation and material to draw your comics on.
And the research that goes into writing is much harder to define and bill by the hour. I can't tell you how many hours I've sat and studied specifics of the Victorian era for my novel, to understand and grasp how to make the hard outlines of a character believable in their setting. I have watched at least a dozen documentaries, taken notes, studied the entire history of Britain in the years 1820-1880, the American Revolution, race relations throughout these decades and so on and so forth. And that is just the part that I don't have to come up with and fit in the story.
It's true that sitting down and writing is not labour intensive, but as an artist you can put your pen down and take a break and not think about it. @LordVincent described how writers 'write' every hour of the day, but that is not a work load to be underestimated. It is a necessity to be able to jam all the little tidbits of story, character and in my case systems of magic and the supernatural in the research I have done and not have it split open like some alien parasite bursting out of history's bosom.
Repeat this times 3, or 4, for stories I have still in wait.
It's easy to think that a writer has a bout of genius and suddenly sprouts story from their ears - and maybe that is the case for some of these lucky writers that ask for collaborations - but in my case ideas are grown over years, and by the time I would ask for an artist on a story, I will probably already have laid 6 months to a year into the project.
So yes, this thread does make me feel insecure, since it tells me that all my work is still worth less than the artists', and I as a writer should bow before you mighty gods that would grace mere mortal me with 30%.
I have spent long nights working too, I even took university electives to research the topics I needed and to improve my writing. But you don't see those because I don't come out of the room with a drawing. Don't get me wrong, I do it with love, and I love writing, but it's not a rewarding field to begin with, so to devaluate it so easily as a side-job I don't feel is right.