Most of those that you mentioned are considered professions that require some sort of specialized training and/or board certification with some sort of oversigh

Most of those that you mentioned are considered professions that require some sort of specialized training and/or board certification with some sort of oversight body. While they don't engage in collective bargaining, they do serve to control who is allowed to practice a given profession, thereby limiting the pool of qualified individuals. While different from a union, these professional bodies serve some of the same functions in the end (raising wages, establishing standards, etc.) As far as why software engineers haven't unionized, I think this answer at the top about sums it up https://www.quora.com/Should-Silicon-Valley-software-engineers-unionize-or-otherwise-band-together-in-order-to-extract-for-themselves-more-of-the-billions-of-dollars-being-created-in-the-region https://www.quora.com/Should-Silicon-Valley-software-enginee...