Very much in the historical concept of labor vs. professionalism. Historically, in the USA at least, unions were generally viewed as part of a "lower" class of laborers that had to band together to create political power to deal with "management" in terms of working conditions, salaries, etc.
For engineers to get organized into a union, you would be diminishing the status, class level, and respect of engineering (at least in the historical socialogical views).
Unions were somewhat arrayed with anarchists (Chicago) and with socialists, communists, etc. (but not always) and this was totally contrary to the "other" side of society made up of upper class careers like doctors, lawyers, managers, businessmen, etc.
There is also the concept of professional ethics that governed architects and engineers over the years whereby one engineer would be prohibited from competing with another engineer by underbidding services or by politicizing the selection process.