True, it's why in industry artists are often paid an upfront flat page rate when working on someone else's (the writer/owner) script... but I don't think a lot of these collabers are aware of that when they pitch or spout about royalties. You'll split royalties when you're equal creative partners, and you need a contract etc for that or it'll end in tears.
Writers are more prolific, that's just a fact. My current collabing partner has finished three scripts in the time it's taking me to draw their 12 page comic, but the amount charged if we were charging would reflect that. My pagerate would be about equivalent to their full script rate for twelve pages. It doesn't make one form of art more valid than another, though, which seems to be an aspect of that whole debate and my personal bugbear.