Because they distract people's time and effort from actually working on worker's rights, and always derail conversations about rights for all workers into how everyone else should unionize.
Actually working how? You can have all the conversations you want about what you'd like, but how are you going to get it without joining forces - which is the definition of a labour union?
Of course union reps would say that, it creates more jobs for union reps. But it's bullshit.
As I see it, it's the pro-corporate bullshitters who want to disband labour unions, precisely to put employees into even weaker position compared to employers. The whole anti-union meme is part of that attack on the working poor, a classic divide and conquer tactic. It preys on people's self-importance by making them think everyone will get what they deserve in such a situation (and of course they deserve more than Joe Average). But for the life of me I can't figure out why anyone would think such a situation would make worker in general better off, nor have you offered any concrete reasons.