Let's face it: as far as we know, the door lock manufacturers also have a master key to all our houses. The schematics and design of the lock are not publicly available, and most people lack the skills to know if the schematics they are looking at are secure or not. It's the same with an OS. And while I *could* take the lock apart and figure out how it works, I still wouldn't know if my particular lock were secure or not, because I have not seen enough locks to know if this particular one is good or not.
Anytime this condition arises, we replace our own lack of knowledge with a trust in experts. We have to defer the judgement of security worthiness to an expert we trust, in which case we are again disinter-mediated from knowing if the lock is actually secure or not. We all trust *someone* with very specific knowledge to help us make decisions, whether that be medical, scientific, security or otherwise, and in each of those cases, we can find examples of where the expert has let us down.