Earlier quoted context omitted.
WGA as a co-op obviously wouldn't make sense. The studio itself is what should ideally be a co-op.
Or the WGA should start creating their own movies, of course.
1) Distribution is where studios' value-add really comes through: they're the ones who have contracts with the big theater chains (or, in the case of tv, often own the networks+streaming services) and are in control of the dates/weekends/IMAX exclusivities/etc.
2) Financing - Movies and TV are _expensive_. It's not just the high-profile actors or marketing budgets – indeed, many high-profile actors will take huge pay-cuts to work on passion projects – but, if you were to take away those costs, and for sake of argument let's say that everyone acting/shooting/editing the movie is working for free, there's still huge capital expenses to be outlaid: location fees, permit fees, cameras (even a single indie-film camera can be upwards of $50k), sets + construction, props, editing+vfx software, food, transport, clothing, makeup, logistics and infrastructure including things as fundamental as electricity and plumbing if you're filming anywhere that's not a backlot... It takes a huge capital upfront outlay to make a movie or tv show that looks anything like what we're used to seeing on tv and movie screens. Studios can (a) provide the upfront cash and deal structure such that financing the productions is feasible in the first place (b) re-use and cost-optimize everything from sets/space/cameras/software/support-staff across productions where it makes sense, so a show or movie isn't having to re-invent everything from the ground up each time.
The closest right now the WGA or SAG can possibly be to "creating their own movies" is working with an indie studio, like A24 or NEON, who are still very much studios, but often with a more "can-do" and "per-production" approach. These studios also settled with WGA and SAG very soon after the strikes began, and were granted strike waivers so they could continue production, while the Disneys and Netflixes were picketed.
In short, unlike software engineers where a single dedicated engineer can pretty much create an app or business mvp in a weekend, writers and actors deal with the constraints of the real-world.