On the other hand, if a union imposes some broad agreement on the company under threat of strikes, I can't convince my company to break that agreement on my own because I'm not more valuable than the entire rest of the company combined.
A union's interests are probably not completely aligned with mine—they're focused on protecting the majority of employees, who are probably not as valuable as me—so forming a union could very easily lead to me getting less of the things I want. For example, if a union convinced a company to pay/promote based on seniority rather than performance, that could be good for most of the people in the union but bad for me.