Solidarity with the users of the systems you make. Collective bargaining is useful for more than salary and perk setting. It also gives you a front to push back against unethical project requests by management. Without a Union, you just get shluffed out of the way and the work gets handed off to the next sucker. With the Union, things like Google's "Don't be evil" can be more effectively enforced from the bottom.

Capital hates organized labor specifically because they have to worry about the risk of collective action, which changes the risk calculus to favor less extreme profit generating opportunities at the expense of minimizing poking the workforce the wrong way.

You alone can be ignored. You and your Union brothers, in great enough number, cannot be.

It's ok to not have an open mind. You can just say that.

You haven't given any good reasons why unions are bad that I and other people haven't already given counterexamples for.