Earlier quoted context omitted.
For me, it's a privacy concern. Closed source means only one company is fixing vulnerabilities, whereas open source invites security researchers to find and fix issues quicker. Fewer security gaps == less privacy risk.
I've heard that argument before, but has that actually been demonstrated? Ability to look at the code (especially in the age of AI) means that security researchers aren't the only ones who can look for bugs. For example, look at the bugs like copyfail that AI has recently uncovered in the Linux kernel.
There exists a danger that very good hackers will be able to find vulnerabilities by looking through the source code, but very good hackers find vulnerabilities without source code anyway.
Consider the following: Would you rather walk down a busy street hundreds of other people walking by, including police officers, good samaritans, and maybe people who want to do you harm, or would you rather walk down a dark ally with only four people in it?