I would need someone with a law background to comment on this (is Courtney Milan around?).
However, actors are also indie contractors, not studio owned, and are able to unionize. Authors are several decades past the point where we need collective bargaining rights.
Publishing is suffering because authors are criminally underpaid (so are many editors and most publishing staff).
By underpaying authors we make writing a job limited to the people who don't need money or to people who can work multiple jobs. That's not good.
We're losing (or deliberately excluding) diverse voices in publishing by keeping author pay and the cost of books artificially low. Books are not a mass market production everyone reads, they are a luxury item like a movie or a theater production. There is a secondhand book market for a reason.