Software Engineers don't like the restrictions on their freedom that unions bring about, and they tend to be more individualistic. They would like to get paid on their ability, not the same rate as every other developer. If they were Union they'd be stuck where they were if they had the usual defined benefits plan, unlike a 401K that they can move around without penalty; and Software Engineers probably tend to have job changes more due to the nature of project work, with each project having the potential to be quite different that the others.

One Software Engineering gets more ho-hum and standardized, then you'll probably see more Unions involved.

Software Engineers don't like the restrictions on their freedom that unions bring about, and they tend to be more individualistic. They would like to get paid on their ability, not the same rate as every other developer. If they were Union they'd be stuck where they were if they had the usual defined benefits plan, unlike a 401K that they can move around without penalty; and Software Engineers probably tend to have job changes more due to the nature of project work, with each project having the potential to be quite different that the others.

Once Software Engineering gets more ho-hum and standardized, then you'll probably see more Unions involved.