Everybody can check the source. ... But because most users/people generally are not qualified to do so,
Why do people keep saying this? It equates "I can't verify" with "no one can verify".
Not really. Regardless, when you take the whole sentence, then it definitely doesn't ring home close. As it equates "I can't verify" with "it takes some time, if ever, before I am sure whether it is safe or not".
As long as there's the possibility of someone verifying, people who can't personally verify have much better reason to trust it.
Not at all. For one, I would have to know IF it has been verified, and then WHO verified it and whether they have done an adequate job on it. Without knowing that, there is no real reason to trust it at all.
There's a parallel here
Believing before seeing/confirming and assuming it will happen one day is more like a religious stance IMO.
And second, it's more likely that closed source programs have a real company behind them instead of someone's OS IT hobby project. Either way, it's generally easier to trust people who have more to lose than those who don't, open source or not.