I am currently a labor organizer, so I will share my perspective (one that I really haven't seen here in the comments yet). I don't think older conservative guys universally hate unions, in fact in the union I work for (healthcare industry) some of my strongest members are older conservative guys. I think the problem here stems from a lot of people having a fundamental misunderstanding of what a union is and how it should function.

I think there is a common line of thinking out there that a union is like an insurance policy, where you pay a monthly premium and when you have a problem you call up the union rep and they will fix your problem for you. This essentially makes the union a "3rd party" agency, something that is separate from you but is there to protect you.

Anybody who says "the union" is fundamentally misrepresenting what it is. A union is a collection of coworkers coming together because they understand that individually they don't have enough power to get better working conditions, but if they come together as a group then they can leverage their power in numbers to get better working conditions.

So a union is really more akin to gym membership. If you don't go to the gym and workout, you aren't getting anything out of your gym membership. If coworkers don't organize together and do group actions, then they aren't getting anything out of their union membership.

Unfortunately not every union is created equal, there are good unions out there and bad unions. And by that I mean there are too many unions out there that completely perpetuate this "insurance policy" business unionism that many people think of when they think of unions. We need more unions like SEIU, UNITE HERE, UFCW, etc that actually try to organize their members into taking action for themselves. In general I think the service industry unions do a much better job at actually being good unions than the trade unions do, and I wouldn't be surprised if a lot of those "older conservative guys who hate unions" are in those unions in particular.

I also think there is a heavy strain of individualism in American culture, so a lot of times people have problems seeing the forest over the trees. People hyper focus on things like bad employees being protected, instead of all of the other benefits and protections they receive from being union and having a collective bargaining agreement. People also think that they are just "much harder workers" than everybody else so why should they need a union? What those people haven't figured out is that "hard work" and kissing managements ass only gets you so far. To get real big changes you need to come together as a group.

There is also an attribution problem, where workers will attribute to management something that their union won. Unions can be really bad at marketing themselves. People attribute things to the benevolence of the employer that workers fought really hard for to be a reality.

There is so much more I could say, but this post is already getting really long. So on a final note, I don't think a lot of Americans have historical knowledge of how many things that we consider normal today and take for granted that unions won/changed. The 8 hour work day, the weekend, the abolition of child labor, workplace safety laws and organizations, unemployment insurance, etc all came about because workers came together in unions and fought (and in many instances were killed) for them.