You can hire a non-union plumber. There isn’t usually much of a price difference. Where I live, you can easily find a non-licenced plumber (called “moonlighting”, usually done by apprentices of licenced plumbers). A lot of people prefer not to since you’re on your own if something goes wrong.
Plumbing requires skill, particularly for difficult jobs, and also requires advanced equipment to do such a job in a reasonable amount of time, such as special cameras to inspect a septic tank or drain line without having to actually cut into it.