The free software license specifically gives the software an extra advantage in that changes to the software must be shared openly, if distributed as as binaries.
I think part of why this social engineering works so well is it takes advantage of that "many eyes" trust, where people are prone to delegating the responsibility of checking to the community and not do due diligence on themselves. I know I'm susceptible to it if I see a Github repo with more than 10k stars on it.
By now (imo), the entire web is gamed and no number can be trusted, I operate completely on a qualitative basis rather than quantitative, basically the only way I can get something out of the web. Ignore all and any numbers as any indication of anything.
It's not perfect, but surely it's easier to audit for malicious code than closed source.
Also, there is no shortage of volunteers looking out for code changes in established open source software. I think it's fair to exclude software that is very new and/or that has no users, which may be closer to equal footing with proprietary software.
Even for established proprietary software, you get volunteers watching out for changes in releases. Though, far less than open source, and more reserved for people who know reverse engineering.