Collective bargaining is pretty much the only tool workers have to even begin to correct the power differential between employer and employee. Ideally, it allows employees and employers to negotiate on more equal terms than is otherwise possible.
No, it isn’t. There is the individual threat of quitting. And there are the courts.
1. An individual quitting at a company where they are one single cog doesn't impact the company in any measurable way.
2. The court system in the west gives an asymmetric level of power toward the corporation, an individual has neither the time, the money, or the ability to navigate an ongoing protracted lawsuit with a corp that dwarfs them in all three aspects.
I've personally used both to extract concessions from employers.
In any case, my general point is that we need more protections in law for workers. Unions are a good way to distract from that.
I genuinely don't know why employers do not hire non-union employees in such circumstances; I think most of us assume they "can't", but would like to hear from someone who actually knows.