I already replied to the parent question explaining how authors of short fiction do, in fact, make money when their work is adapted.

The WGA is a union whose members pay dues based on their writing income. The guild is incentivized to maximize their membership, and to maximize the earning of those members.

I'm an associate member of the WGAw--someone who's worked in the industry, but not done enough work yet to qualify for full membership--and I've never encountered any of the exclusionary attitude you suggest.