That's the whole point of joining a union it turns a unilateral contract negotiation, where one party has no leverage, into a bi-lateral contract negotiation. The worker's leverage in the negotiation is to withdraw all labour halting production. The downside is that you don't get paid while you are on strike and it only works really well in industries where there is a labour shortage or it takes time to train up new workers who need experience and qualifications that are hard to obtain: pilots, train drivers, doctors, police all have very effective unions in the UK at least.

The difficulty that the amazon workers face is that they turn up and a screen says pick up these things from these boxes and take them over there, put them in other boxes, stick postage labels on and put it in the back of the truck. It doesn't take much training to do, so workers who unionise are very vulnerable to being fired until enough have joined the union to hold an effective strike.

Unions are always going to naturally form in a pure libertarian society where the only rules are freedom of contract. Usually they are restricted by government regulation.