Even in cases where there is some leverage (due to work being skilled/scarce), I'd often like someone to negotiate on my behalf collectively. I'm not an expert on employment law, on "gotchas" in employment contracts, or on what my rights are under local law, and I don't really want to become one. And I don't want to fight every individual battle over and over again when many of us have similar preferences. For example, why should I have to fight every company over noncompete clauses and "we own all your weekend IP" clauses? Even if I could win, this is a huge duplicated effort if every one of us has to argue this same point, and I'm not a contract lawyer or professional negotiator in any case. I'd rather delegate my negotiation authority to some association of technologists that negotiated at least baseline employment conditions.

I guess an alternative model is to take it really collective and just have the state pass laws mandating working conditions that all employers have to follow, that way nobody needs to negotiate those conditions. That's the approach California took with just banning noncompetes by statute, for example.