I'll do unpaid if it's for charity. Usually charity zines and the like is fanworks anyway so I never have to really worry about copyright. But, most of the unpaid collabs on here that aren't charity or some sort of cross promotion, are completely unaware of the time commitment art takes and are extremely demanding (and they'll have REALLY intense deadlines too, of 30+ panels a week and it's like good lord you aren't paying that person!) I don't want to work with a controlling bossy person.
And like, I'm an adult, so in my mind, forking over a couple hundo is what you would pay to fill your car that month with gas. It's what an artist deserves, so they too, can fill their car with gas. If you refuse to do even give as much as gas money, then it looks like you are either a child (which is fine, but I'm an adult so it would not be appropriate me to work with an underage person that closely behind private DMs. Some stranger-danger there, so I don't respond to those collabs.) or, that you are an adult but you have absolutely no confidence in your work to invest in it, and that you are actually looking for an artist to fix your project for you.
And then finally, many of the collabs are kind of...poorly written, as just a collab post. So it's like...you want to be my writer and you can't write a post? And I know it's because of the age of the collab makers, many of them are teens. But, they are trying to convince seasoned artists that they're a better writer than them. All artists on here also write and are sitting on like 3 or 4 scripts at all times, that's why we got into webcomics. So instead of writing a boring collab post that reads like a grocery list, write to get me interested. Convince me my script isn't anywhere as good as yours, and then I might look twice at it.