For me, it's this thing of seeing writers come to Tapas looking for an artist to "collab" with them on doing a webcomic- when most artists here are already doing their own projects; there are a few artists here who want to work with a writer, but not many. They would better fare looking at sites like Deviantart, Tumblr, or Instagram.
Second, I do agree with the general consensus that a collab is a joint venture between 2 or more parties; if a writer already has a script & characters and is just looking for an artist, then it's not really a collab- it's for hire. The artist has no equal say in any of it.