Ah, but you clearly don't darn your own socks or repair your own clothing. If you don't have the seamstress skills and tools to take care of your own clothing, if you just throw away your clothes when they get a hole or you pay a tailor to take care of them, then I don't think you're the kind of person any business would want working for them. The lack of initiative of not becoming an expert in absolutely everything is glaring and you would be a drag on the company, you'd let the company down, be more trouble than you're worth because you're not a superman who is an expert in everything not even related to what you do for the company.
I'm not sure why you have gone down this path of comparing engine rebuilding and seamstress/tailoring expertise to negotiating for yourself in the workplace. No one claimed you must be an expert in everything.
What you can do, with little effort, little time and no tools is work on your self esteem and communication skills to be a better advocate for yourself and more effective in the workplace.
You stated yourself that:
I'd always prefer to be in a union. Negotiating stuff in person can be very intimidating and I'd rather someone else do it for me.
So work on that. No one will argue on your behalf better than yourself. You could have way more to offer and be of much more value to your company than the rest of your team yet you are willing to settle for an average negotiated for the group?!?! That would infuriate me. You might perform really well but the rest of your team drags down your department and you want to settle for that versus having a meeting with your boss to discuss how great you are at your job?!?! Or perhaps you are the drag on your team and that is why you are just happy getting the negotiated average?
Why don't I just pay you to negotiate for me then, and instead of going to Toastmasters I can do something fun instead.
Then when someone tries to offer a way for you to work on your skills you dismiss them because you want to do something fun. This clearly shows all of us that you cannot be bothered to invest minimal time and effort to be a better advocate for yourself. I went on to say that if you are unwilling to stand up for yourself in the workplace then you probably won't stand up for your ideas in the workplace either and then you probably aren't a good candidate, at least in a team environment where collaboration is key.
Pretty simple stuff here but YMMV, if you are too intimidated or bored to be an advocate for yourself then don't ever complain you don't receive the compensation you feel you deserve. A union, at least in terms of compensation and benefits, is not an advocate for you, they are an advocate for themselves and then the collective, remember that. You are nothing to a union besides a source of income to support their power.