EDIT: removing the crud
beyond supply and demand, this is because most writers come to the forums looking to employ / commission an artist, and most artists come looking to be employed / commissioned*. the writer wants you to draw their idea. the artist wants to draw your idea. imagine if you had two stands, at one the guy called out 'give me free icecream!' and at the other, the guy called out 'FREE ICECREAM!' who's gonna be more popular?
*this isnt always the case, but usually calls for equal collaboration dont get the silent treatment that unsolicited calls for free / spec work do.
if you cant afford to pay someone to make your Thing, you dont deserve to make your Thing. whatever you put into it prior to the make is irrelevant.
that is called spec work. making someones comic for free because they COULD get big, thats extremely-unlikely-to-be-fruitful spec work. you should never, under any circumstances, be you an artist, a writer, or an accountant, do spec work.
you should listen to this, itll be enlightening to you: