But proprietary software vendors have an interest in keeping things secret, while open source code can be reviewed by people who have an interest in making prob

But proprietary software vendors have an interest in keeping things secret, while open source code can be reviewed by people who have an interest in making problems known. With your logic, you might as well say that freedom of information laws are useless because nobody can review all of the government's documents by themselves. The point is not that anyone can review it all by themselves--it's just that it's more reasonable to trust things that the public can verify rather than unsubstantiated claims by someone who has an interest in keeping things secret.