As everything, it goes in waves. The first people who were pro-union, were the ones who had 6-day work weeks, probably started working when they were ~13 years old (at least in the country where I was born) and had basically no work safety at all. Generation after generation got better and better working conditions, until modern times where many don't see what a union would even do for you. Probably lots of unions are riding their history as well, and have become complacent.

But the more workplaces start abusing their employees, the more pro-union workers become. So that's what we're seeing today, people don't start to become pro-union just because.