OK, so you can negotiate your own rates, and get a good deal. That suggests that you're a pretty good negotiator.
But are you really the best (non-managerial) negotiator in the entire company? Statistically speaking, the odds of that are low. And with a union, you could have the very best negotiating on your behalf. Yes, you'd have to give them all the information—but you have to do that now with your managers, in an adversarial setting, and you could instead do it with the negotiator in a friendly meeting where they're actively working to help you get them the information to get you the best deal.
Or, maybe you really are the best negotiator in the area. Wouldn't you like to get some extra pay from the union to be that guy above, and use that negotiating skill on behalf of the rest of the senior workers? (Don't think they'd pay you extra for that? Well, you're the best negotiator around—make it happen!)
Or maybe you not only genuinely believe there's no one who could possibly be better than you at negotiating your compensation, but also genuinely don't care that other people have the opportunity to be paid what they're worth, too. In that case, all I can say is I Don’t Know How To Explain To You That You Should Care About Other People.