This is exactly what these workers are doing. They're unhappy with their contracts, so they're trying to negotiate better terms.

Normally, you negotiate your contract because your company wants you, and finding someone else that would agree to the contract is a non-trivial amount of effort, so they'll hear you out. In these people's cases, all they'd hear is "lol, no" if they tried to negotiate individually, because they'd have virtually no power. As a result they're trying to find leverage in numbers: We want the freedom to negotiate all our contracts to our advantage, otherwise we're striking.

Allow me to put things differently: There is no law that says that Amazon must accept whatever terms are put forth by these unions. Amazon can, if they wish, outright refuse their terms and tell them that the contract is as-is and will not be altered.

While I'm not saying this is your case, I am puzzled at people who argue against unions with your last line, because it really does cut both ways. "If you are unhappy with a contract, don't sign it". Well, a union is a form of negotiating said contract, and to the companies being proposed those terms: "If you are unhappy with the terms, don't sign it". Makes sense, right?

Law of nature says people have to eat. Amazon warehouse worker is a low-skill, low-education job - how much bargaining leverage do you think an individual has in this situation?

I agree with what you're saying, and I'm very skeptical of the argument you sometimes see that everyone ought to unionize out of solidarity. But some workers don't feel they can get a contract they're happy with on their own, and for people in that situation unionization makes sense.

That’s essentially what a union is: a mechanism for collective negotiation of employment contracts.

People don't necessarily have a choice. If that's the best contract that you can get then there is not much that you can do about it.