There's more to a union than just negotiating on behalf of employees. Unions have quasi-government status wherein the National Labor Relations Bureau can just step in and force the company to make business decisions. Furthermore, unions aren't optional in a lot of states. If you want to work a certain job at a company, you have no choice but to join the union (or at least pay union dues).
Something like the National Press Photographer's Association [1] is a valuable organization. They provide resources like contract templates, and can help if a customer doesn't pay the agreed royalties and whatnot. But membership isn't mandatory. A software developer's association and a software developer's unions would be to vastly different groups.
1. https://en.wikipedia.org/wiki/National_Press_Photographers_A...