Because employers have the power. Without the legally protected status as a union you have no legal right to protest the conditions of your job. You have no right to refuse to work under dangerous conditions. And employers are free to retaliate against workers for even talking about unions or talking about reporting the dangerous conditions. How are you going to get people together who are all desperate for money and get them all to agree to go on strike and then get other people to not come in and take the jobs. One way used to be to call them scabs, make everyone else hate them through propaganda campaigns, and hire the Mafia to beat them up until they quit and no one else would take the jobs. It wasn't until the NLRA that unions were protected.

But conservatives have turned anti-worker now due to their reliance on corporate donations among other things. And they have spent decades making unions look bad, saying they're just criminal organizations, and calling them communism. So not enough people are going to feel sorry for the striking worker or hate on the scabs enough to pressure the companies to give in to demands.