The way it works with my partner is that the collaboration was born both out of necessity and shared interest. Both of us work on both aspects, if it makes any sense. I might suggest ideas but not all of them go in because not all of them are good, but I also assert when he's struggling to work through a scenario that as an audience, I would really hate certain things being done. So I also push him to seek for better solutions. For artwork he'll have the general idea of how something should look that is ultimately based on a practical, mechanical level. I'll be given a list of materials and try to design within these parameters, while keeping in mind general aesthetics. How do I make it work but look good, and keep true to the original idea? (These are the kind of challenges I personally enjoy.) We'll go through revisions for these designs, but I'm still the ultimate authority on aesthetics just as he is on the story. Designs and art do come largely from the writing first, if they're so off-base they will completely ruin his plot so he's trying to ensure there's no logical errors.
Over time you get a really good idea of what the other person wants anyway so it's not as time consuming to figure out as one might think. For this sort of thing to work though, both people have to have assertive qualities. You have to be able to tell the other person when something is bad, and they have to be willing to accept the criticism. Without this there is no foundation for teamwork. Which is why I'm not really going to jump on the sort of "pitches" people offer in the forum. There's absolutely nothing to tell me they have these qualities as a person. It's also not necessarily "co-authoring" as much as it just ... I dunno... world building together? But the point is for me that if I don't get any say at all at what goes into the story or even on any design level, that's where people expect to be paid because there's no emotional investment in the project anymore. This isn't bad, but this is what I consider the "business" version of collaboration models.
Ownership is really just "Written by, Illustrated by" at the end of the day. 90% of the writing is his work, 90% of the artwork is my ideas so to credit it any other way would just start getting into semantics. Co-authoring is really a whole other thing in my mind compared to what we're doing if that makes sense. Co-creating is probably the better word. But again I'd have to stress, situation like these are rare and only come from both people enjoying each other's work. So in that regard ownership, profits, money all this kind of stuff is a lot less of an issue because it's not the point of why you're working together. Those topics would be on my mind a lot more when it comes down to working with someone I don't trust and don't know. But that's also why I don't put myself in that situation, because there's too much risk, even if I'm getting paid. I guess the other part is I don't trust an indie author to have the sort of funds that would adequately compensate for my time. The person I work with I know in real life so there's a lot more accountability and trust that was there in the first place. I don't think unpaid collabs inherently only exist to take advantage of artists.
I just think most people pitching them aren't writers, are either young and/or incredibly amateur, and legitimately, naively believe that promises of fame and fortune are enough to satisfy compensation for the other party. I guess to TL;DR it, they don't know how business works. Or otherwise don't even care that both parties need to mutually benefit.