For some people, I think it's actually exciting for them to think about how independent they are and how nobody can spy on them or trick them out of money because they consider themselves to be so worldly and knowledgeable about such things. And maybe they are compared to their families or the people they deal with during their day-to-day. I've known plenty of people (myself included at times) who get some of their feelings of self-worth from this sort of thing. But because it's more of a fun or emotional or ego-boosting thing than a rational thing, they'll often make big bold binary blanket statements about all the products they refuse to buy or web sites they refuse to go to or companies they refuse to patronize. And that's all great, actually, as long as it's rational and based on facts rather than some narrative in their head. We need more skeptical people, we always do.
But it's not always rational, and sometimes it is more like they're LARPing through some exciting narrative about the world, and in that narrative they're the heroes who know stuff that the normies don't. They're more like survivalists living an adventure in their head than rational people making rational choices. And that's where people can also get into trouble, both for having a false sense of confidence about their ability to avoid risk, and for giving others bad advice based on excessive confidence or rose-colored glasses about the same things.
I'm all for better security practices based on ever more openness, but I agree 100% with your example: people are kidding themselves if they think just because they run binaries built by other people from open source, it's almost as good as if they had personally inspected every line of code themselves. And when people are making those sorts of utopian statements, it's generally coming from the LARPing survivalist narrative part of the mind rather than the skeptical rational part of the mind and should be viewed with that much more skepticism. Not rejected, but just considered with an extra dollop of skepticism, because they hopefully at least mean well.