They are mostly different due to the market where they work in being very different. This is a good thing. No point in trying to put the exact same organization everywhere.

For lawyers much more of them work for small law firms as average us law firm size is 3 people including the owner(s) vs something like auto unions where the employers very often have 100k+ workers. Also by definition all of them are experts at law so do not need one of the most important benefits that a modern union gives its members (lawyers).