Just because you're paid well doesn't mean others are not being mistreated

FTFY
without unions there could be a huge salary disparity between devs in the same role, in the same company, even in the same project. I've personally witnessed more than 2x, heard about even more.

Sometimes it's more than justified with individual's performance and impact, sometimes it's not. Some people are just better skill-wise, some people are better at applying pressure on their employer, holding business-critical knowledge hostage or simply negotiating.

Point here is - while unionizing might make things better on average, there would be a very real pushback from people who are benefitting from current system and this is not necessarily management. For management in some cases it would be even a net benefit, since they don't have to deal with primadonnas and someone tying things to themselves just for leverage.