Ask HN: What is stopping people from unionizing in Tech Big 5?

First, I would like to start with saying that I don't heva a horse in this race : I live in EU country and I am quite intrigued in the latests news coming from Google related to union busting and latest from Kathryn Spiers. I am not looking for arguments about why she deserves or not to be fired.

So my question is why don't her and other people in Google who want to form a union(as it's their right if they want) just start one? Allegedly by media, at least 5 people were fired for that in the last month. What's stopping those 5 people to have formed an union before they were fired? You start with 5 and get in as many willing people and voala. Isn't that how it works?

Is there any requirement in US about percentage of workforce that has to be in a union? Can you just start something with like 50-100 people(I'd assume that there is at least that many that would be willing to join if someone starts one) and then grow it slowly, using legal ways and start building channels with management.

P.S I'll try to state that I am not looking for pro or anti union oppinions. Forming an union is their right, and they seem to be wanting it, so my question is just why wouldn't they just start one using legal ways and means.