I’m not sure that is fair.
Independent schools are very different from the state sector where you wouldn’t have issues like fee remission or your accommodation being mixed up in how you get paid.
People can be frightened to strike or even of joining a union; they might be complacent about their job security or they might be politically averse to striking.
Independent school heads can be remarkably naive about employment law and have little experience of union activity. They might ask things of their staff that they shouldn’t, and go unchallenged.
We don’t have the STPCD - and never have had anything like it, and we don’t have people chirruping up in the staff room that ‘They can’t ask you to do that!’, so rights are not defended or widely talked about.
No. It is not because people are thick. And you don’t tend to Google what you don’t know, so that isn’t the answer, either.
Try refusing to cover for your colleague in a boarding school (when your children’s education is paid for by the school and so is your home), and see where that gets you.