I think a lot of software engineers (especially those who post on HN) think of themselves as top-1% Captains Of Industry, who would never benefit from a union. "Unions only help those guys lower on the totem pole than me!" says every software engineer out there, so they disregard it as something that could help them. We all think we are Temporarily Embarrassed John Carmacks.
That doesn't explain why doctors that see themselves as top earners didn't have a problem banding together. Social organization doesn't require unions in socialist/communist sense. It can also be accomplished through other professional organizations like AMC.
