Because even for FOSS stuff, unless you are an expert on all levels of the stack you will not be able to assert there are some hidden exploits disguides as perfectly safe code.
And if you have to rely on third parties to assert that for you, then you will have to trush their honesty and technical skills to be able to assert such statements.
So there is only hope that all players are experts, don't do any mistakes, keep being honest, and exercise their certification for every new release for all products that make up a standard installation.
Because even for FOSS stuff, unless you are an expert on all levels of the stack you will not be able to assert there are some hidden exploits disguides as perfectly safe code.
And if you have to rely on third parties to assert that for you, then you will have to trush their honesty and technical skills to be able to assert such statements.
So there is only hope that all players are experts, don't do any mistakes, keep being honest, and exercise their certification for every new release for all products that make up a standard installation.