As a former server-side game dev I was always supportive of my co-worker's rights (QA & Art in particular have a _terrible_ time) and I think this is where pressure to unionize is likely to originate.
Back when I was actually interested in entering the industry none of the companies I found would even talk to devs without games industry experience, generally major shipped titles, too. That could be part of their problem.
I would have thought it was the opposite, and to produce a AAA title you'd need the very talented people - the experts - and they have more leverage to negotiate.
In many cases, unions increase the value of labor by constricting the supply of labor. Unions can often be difficult to join. If there is a overabundance of labor, then the union has little negotiating power.