Operating systems need hardware to run; hardware needs drivers; many drivers are closed; backdoors are easier to conceal in code that is closed and unavailable for auditing. That is not something everyone could do, but that doesn't mean nobody could do that if given enough motivation and resources.

If you buy a system that requires closed source drivers then that's your own choice. Nowadays far and away most consumer hardware will work without any closed source drivers. Except of course the big N.