Some of them are genuine psychopaths only concerned about having the most docile, least demanding workforce they can squeeze the most out of. That said, there are good reasons to be against unions, even from the perspective of the worker. Collective bargaining, without some union that you pay dues to that makes some guy other than your boss filthy rich, is possible. Unions also have connections to unsavory characters like the mafia and actual communists, and people have ended up dead for crossing them. Even when they give campaign funds to your average milquetoast centrist, your dues support things that will erode your rights and wages faster than the union itself disappearing, because union leaders still support Democrats who have gone all in on cheap third world labor.
Unions also kind of screwed themselves when they got extremely lazy in the mid-70s through the 80s. Some of my family comes from a former union, now rust belt town with a lot of factories making parts for all of the major American auto makers. They often bragged about how they would strategically place a bolt or something in two sheets of metal that were to be welded together in such a way that it would become dislodged at some point after the buyer drove off the lot and there would be an inexplicable rattle for the life of the car. There were other (mostly) non-dangerous acts of sabotage, but that's the one I remember most. They did it because they thought it was funny. You never bought an American car built on a Friday because they were getting drunk by lunch. You never bought an American car built on a Monday because they were hung over. They couldn't be fired when caught. When my parents had their first kid in the early 1980s. my dad bought a Honda because with all the other things 80s parents had to worry about like daycare centers run by satanic pedophiles, they wanted to make sure there was one less way for the kids to die.