Taking a step back from the points that everyone is making here (some I agree with, others not as much) I just wanted to add some of my experiences as an example.
I'd like to believe that I'm somewhat of an established author but when going into a collaboration, what's taken at face value is honestly what you bring to the table.
Just like how artists have portfolios for professionals to look at, I clearly state my "portfolio". An example of a writer's "portfolio" won't usually be just a list of manuscripts/novels/poems/etc that they've finished but also a comprehensive statistic of how many readers or followers that they have. This way, just like how artists are trying to entice their viewers with their artistic skills, writers can entice by guaranteeing at least "x" amount of initial viewers that will be interested.
I did a short slice-of-life comic called TurtleDays and since I can't draw for the life of me, I garnered the attention of various artists to draw the scripts that I wrote for me.
This worked for several reasons:
1. The commitment was just one short strip most of the time, with the option of coming back if satisfied.
2. I still paid them (albeit usually a discounted amount).
3. I promised a shout out to the artists' original works so that viewers from the comic strip of my title were easily able to go to the artist profile of the one who actually drew it.
Result:
I was able to get a large number of interested artists, many of whom were willing to do it for free. Why? Because it was low-commitment and a way for them to garner new readers for their original works.
When I posted on this forum about wanting to hire artists for this, I stated how many people are subscribed to my works and how many views my novel/comic has, not because I'm bragging, but because it's what I bring to the table in order to attract artists of a certain level.
TL;DR
As an artist, your work speaks for itself. No matter how "unfair" it may seem to writers, it's a lot easier to gauge the skill level of an artist by what they've drawn.
For a writer, I don't think that just a script and the firm "we can do it" belief is enough to attract an artist. If you really want to attract a dedicated/motivated/talented artist to work with you long-term, the promise of future profit means absolutely nothing unless you can back up that base with proper statistics to prove what you're saying.
I kind of barfed this all out so my apologies for mistakes and the like.