Because it's very easy to get turkeys to vote for Christmas. Turkeys have no idea why you like Christmas or what you're planning for Christmas, they just know what you're telling them. Tell them that turkeys are the most important things on Christmas and it would be entirely spoiled without them, and promise them a handful of feed right now as a reward for voting for it. You don't even have to lie.
What would be the benefit of being in a union as a software engineer? I never felt that I needed one.
A few years ago Adobe, Apple, Google, Intel, Intuit, Pixar, Lucasfilm and eBay all successfully kept engineer pay down through anti-poaching agreements[1].
Unions provide checks and balances against that type of abuse.