Too many people without security clearance can access and modify Linux. In any real security environment, open-source is poison. Period, end of story.
In any real security environment, humans are poison. Period, end of story. This is not an issue exclusive to open-source.
Closed source does not mean obscurity–– and open source does not mean clarity (see OpenSSL, that one Linux 2.6 thing¹, etc).
It's not like being proprietary suddenly means the only security is through obscurity. Why do you think that? Are you just a zealot? Did you not consider that closed source software could be well-engineered and secure? You're welcome to read about some of the security features in Windows NT² (that article is a bit old but still relevant), which are considerably more thorough than (non-SE)Linux (I think SELinux has auditing now, so it's at least comparable to NT).
Now, don't get me wrong, I generally prefer open-source tools (for a variety of reasons) and tend to trust them more, but saying stupid stuff like you are just gives open source a bad name.
On top of that, obscurity is a perfectly valid layer of security. To use a mediocre analogy, of course you want your safe to be strong enough that nobody could break in even if it's in plain sight—but it's certainly not a bad idea to hide the safe as well.
¹ https://lwn.net/Articles/341773/
² https://www.microsoft.com/resources/documentation/windowsnt/...
Is there something more subtle I'm missing?
‘Open source’ is more about the development model (and freedoms) than about the nature of ‘knowing what the software is doing’.
Heck, I could argue that Linux is a black box to most people who aren't well-versed in kernel development. OpenSSL is notoriously difficult to understand. Sometimes huge bugs look relatively innocent¹ even with people looking at the code.
But reproducible builds are a hard problem, so… honestly, I can't answer that.