I think with unpaid collabs, the really frustrating thing is that the writer often comes in with absolutely no writing experience, just "I've always dreamed of making a successful Webtoon, but alas, I have no drawing talent, I tried drawing once, and I can't even draw a stick figure ..." and now it's the artist's job to come in like a fairy godmother and make their dreams of being a successful comics writer, that they've put absolutely no work into, happen.
On the show "Dragons Den" (A show where investors listen to pitches and invest if they're interested, I think it's called Shark Tank in the US?) I always remember a frequent thing the Dragons would say was "What's stopping me from just going and making this at my company?" And I think wannabe writers sometimes have some vague fear of people stealing their idea so are very cagey about giving details... but they're not really dealing with the underlying issue of understanding on some level that somebody else could just go and to their idea better than they could. Like, yes, hiding the idea does stop that happening, but it does it in a way that immediately broadcasts "I'm not a very good writer; if somebody stole my idea, they could probably do it better than me."
See, the problem is, the writer needs to be as good, or ideally better and more confident at writing than the artist. Like a lot of artists, I don't feel comfortable labelling myself as a "writer" because I feel like I don't "deserve" that title....but nearly every comic I've ever made, including ones that placed in national competitions, and practically all the most popular ones, including my current webcomic, I both drew and wrote. If somebody's been making comics long enough to get good at drawing them, they may well have racked up quite a lot of storytelling and writing experience. So if the writer hasn't spent equivalent time writing fics, novels, scripts, games or comics... they're often asking somebody more experienced at writing to let them step in and do a potentially worse job. I frequently find myself disappointed by how lacking in dedication a lot of "writers" are in their study of writing and how little they've actually written.
If you want to be a writer, either pay the artist, or work damn hard at writing and studying writing so that you can impress the artist with your abilities and make them feel confident giving you the job of doing the writing, because most comic artists can write...it's just our secondary skill. If you're still less experienced, studied and qualified in your primary skill than the artist is in their secondary one.... why on earth would they want to entrust that job to you!? If the comic needs to sell in order for me to get money via revenue split, it needs to be the best possible comic, so I wouldn't enter an agreement like that if I didn't have faith in the writer's skill and experience.