Except, there's a good deal of crossover between the two fields. Neil Gaiman is probably the most obvious example of someone well-known for working in both comics and prose, but there are writers known primarily for their comics work who've also written prose fiction (Warren Ellis, Alan Moore, Elliot S! Maggin, Gardner Fox), and those known to us mostly for their prose work who've worked in comics (Harlan Ellison, Samuel Delany, Alfred Bester). And many more.

And there's crossover at the editorial level, too. Teresa Nielsen Hayden worked as an editor both at Tor Books and at Valiant Comics.

In short, I'm not convinced that SFWA has any less leverage in the comics industry than it does in the film industry.