Not just illiterate. How many people here have the time or desire to inspect every app they use? It's a ridiculous proposition. We all have better things to do.
Obviously the onus isn't on every single user to single handedly verify themselves that the software is correct. Do you check your government's finances yourself? So why should they even publish it? By your logic, that is.
The answer is that if it's open then multiple people with differing interests, such as competitors, or even independent organisations such as non-profit auditors, can check the code.
