Sometimes I wonder if these people asking for free work load has ever picked up a pencil and draw a panel. I understand writing a story can be time consuming but honestly, it doesn’t beat hours of research and drawing. When you are done, now you gotta illustrate then color it and then add text to it. It’s A LOT of work. Back in the day this kind of task was split because of the enormous workload that came with it. Most comic company's still do it the old fashioned way.
So back to your question, just like everyone has said, they feel safer. They don’t get to see the people’s faces they are asking and with that, the feel no remorse when they ask for such things. I don’t think it’s worth it unless you really do need to collaborate to gain experience or just for fun.
Another thing I dislike about some people asking for collaboration is that they tend to use the word “my comic”. “My story”, “my idea”. When you are done establishing “I”, it’s “NO LONGER”, a collaboration. It’s just you looking for someone to slave wage for “free” on “your idea” that could maybe make it and “maybe” the artist gets paid.
As an artist I don’t have time for “maybes”, maybe it’s because I work full time and pay bills.
I’m not against collaboration. I believe it’s a great way to network, learn and grow for both sides ( artists and writers)