Combination of people not understanding what goes into a comic, mixed with overestimating their skill level.
I think especially for folks who are younger, they don't have a good grasp of how much work and how much time comics take, especially if they're a writer (or even artist who hasn't actually done comics before). People are mostly conditioned on either professional quality comics that are released on a fast schedule (monthly comic issues, weekly manga chapters, weekly premium webcomic episodes) and just think that anyone who makes comics can produce at that rate, either being unaware of the fact that many of those comics are made by teams of 2+ people and also created as their actual job. It's like, you know, even a manga on the shorter side can hit like 20-30 chapters in a few years, so even if the author moves on to something else afterwards you can still make my story right? No. That 20-30 chapter story will likely take a hobbyist the majority of their remaining life span lmao Exaggeration? Sure. But real talk, my current benchmark was 70 pages in just over a year. 20 pages at ~20 pages per chapter is 400 pages... that's like 5-6 years. No thanks.
Going hand in hand with that is overestimation of skill level. What makes many collab posts even less credible is that the collaborator flaunts their work as extraordinary, and that being the reason why people should want to work on it even without pay and then... more often than not, it's just not lol. Not all artists who make comics are great writers (myself included) but your story had better be something really special for me to spend years working on it rather than using that time to just train my own writing skills lol They just... very rarely are.
My biggest gripe is just that more often than not, the collaborators only want to work on huge projects and never something small in scope. No payment and working with strangers are sketchy enough, but people MIGHT be willing to work on an interesting sounding one-shot proposal or something. But every single proposal is seemingly a 300 fantasy epic and it's just like why? Why do you think people would want to do this???