I don't know why people ride motorcycles, skateboards, or school buses when trains already exist. This is not one organization, these are different groups all trying to accomplish different goals.
>why anybody would run BSD over Linux.
We have our reasons. You have already quoted very valid reasons, the fact that you disregard them speaks more about yourself then the motivating factors of others. You have been given technical arguments, political arguments, and real-world examples of why others think that the BSD groups prefer their OS of choice.
You have already made up your mind that BSD is inferior.
imagine how much success windows + mac would have if they were as scattered and opinionated as the linux community
OpenBSD in particular is an attractive option for some servers, since it has builtin httpd, acme-client (for tls certs), smtpd and excellent documentation of how to get it all up and running with minimal, secure configuration.
While people have a tendency to believe that there is "only one True Way", and it is a popular argument that "X is most popular, therefore it is The True Way." – this is not in fact true for operating systems. Every system will have its drawbacks, and…
Regarding a): Everyone working on one thing does not optimise the quality of the outcome. A rich flora of operating systems encourages experimentation, trying out different compromises and learning from each other’s experience.
it is an attractive option that is less attractive than Linux because it is less popular and has less support
that all accomplish the same thing, with worse polish + support than larger, more successful projects who don't spend time building on top of "slightly different variations" of the same thing
it's undeniable that linux has 10x the support of OpenBSD, and mac has 2x-5x the support for linux in terms of "actually getting stuff done", not tinkering with command files endlessly for no reason
even the "linux is good for infrastructure" excuse is starting to fade away as more and more things get abstracted away with k8s, docker, cloud-config, etc.
yes, by all means, they run linux. they don't (on average) run BSD. anything that runs BSD could in theory run linux and probably be ok overall. linux is not "worse" than BSD at any one thing and is better in terms of community support, performance, etc.
Linux distros however are still moving targets and there's no common stack apart from the kernel which is predictable to test against. This is why its hard for some maintainers to reproduce an issue a user has due to possibly apt-get, rpm, install package or use snap, flatpak or appimage and the app starts to break.
Not a problem for CLI apps, but a problem for testing desktop apps on Linux. Perhaps you want your favourite app being fully supported and tested on your Linux distro of choice. If I know my app works and is tested on Linux distro A,B,C,etc I can confidently target a wider range of Linux users. At least that's how I increase my "Linux userbase".
The fragmentation on this however is another story.