Something that I saw touched on here or there but not outright said yet:
I feel a big part of the problem is that a majority of these would-be collaborators are always interested in hopping straight into some huuuuuuge long, "will probably take like 10 years to finish" type projects.
^ these quotes near the end stood out to me as getting at a similar, yet different point lol. I think that there's absolutely a place for most people regardless of skill level or progress in their art/writing careers to find some sort of collaboration, even if unpaid, but the realistic scope of the project is rarely considered.
Like here's the thing: if you're a new creator without much (or any) experience under your belt, as has been said in this thread and others, it's hard to find collaborators. If someone pops into a thread and you don't have any art or writing samples or past work you can show off, people aren't going to want to collaborate with you probably. ESPECIALLY if you're like "oh yeah, btw, I don't have any previous work but I also want to collaborate on this 500 chapter story, yay! :D". Not happening (probably)
But here's the thing, even if you're that newbie creator without a portfolio if you come in with a proposal like "hey! I don't have anything to show yet but I want to get my feet wet in the comic world! Does anyone want to collaborate on a short 5-10 page story with me??", I'd bet you'd get some bites! Sure there are still plenty of people who would still pass it by, but you would at least have a project proposal that was somewhat enticing. It's like the longer the time commitment, the more not being paid for the work stings, y'know. But something that can be completed on the side for fun in a month or two? Sure why not! Then you can like work your way up.
If you enjoyed working with the other creator a lot- maybe you guys do another collab together. If you didn't like working with them that much- no big deal! You weren't entering into this uber long project, so once it's done you can part ways and potentially try to find a new collab with a different creator but here's the thing: now you have a completed collaboration under your belt that when the new people on the new thread are like "hey, do you have any experience doing this?" you can be like "why yes! Yes I do! -insert project here-".
In short, I just don't think that people making these collab threads know the value of the building blocks and baby steps enough- or maybe don't even know that they exist. Again going to the above quotes, maybe they think that everyone just hops into 1 big project and it's either a hit or not and that's it? Where that's just simply not the case for most successful people. Everyone has to start somewhere. Learn to walk before you try to run.