I know. But the problem is that in reality the only way to get people to audit software reliably is to pay them to do it, so it's not really true as a general principle that open-source software is more thoroughly vetted.
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.