Firing people to hire contractors instead is risky because you need a reason to fire people. The reason could be economical, but one could easily argue in court that if the company is hiring contractors instead, well, it was not the real reason.
The other reason could be personal. The employee is incompetent or toxic, for example, but if you get an offer as a contractor after, it sounds trivial to win a trial too.
I know the word is a bit a taboo, but I’m personally in a union, for scientists and engineers, and I would recommend people in USA to start considering thinking about perhaps potentially slowly improving their rights.