Getting collective bargaining started requires assembling the collective - this is a lot harder to do if the employer can just fire anyone seen organising.

Traditionally bootstrapping into a situation where union organising could happen safely and peacefully required a period of violence on the picket lines. There used to be quite a lot of this in America: https://en.wikipedia.org/wiki/History_of_union_busting_in_th...

These days it's even harder in some ways; the more "virtual" an organisation is, the easier it is for the company to just disband and restart it and the harder it is to picket. It's not clear if the various strikes against Uber by its workers have achieved anything.

(someone is downvoting you, but it's not me!)