Most Linux distros can do that without any external or extra repository.

That is not the same as in base though. For example, the NixOS developers maintain a large chunk code to generate scaffolding around systemd and to build code via Nix. They do not take a web server, fork it, and maintain it (alternatively, write that web server from scratch as is the case for httpd(8)). When I set up that OpenBSD server, I install the base system, place the configurations, start the daemons, and I am set. Not a single line of code runs outside of what is in the base repository.

I realise that the way BSDs do things is very different from Linux, but in BSD land the same people write the kernel, user land, and maintain the ports tree. With this I am not saying it is superior, but it does lead to a very different experience both as a developer and user. Yes, there are some exceptions to this like clang, the AMD GPU driver, etc. But the overall picture is true.

Right, the difference is those daemons are written by other teams/people who have various experience/knowledge/security requirements for their software.

The daemons in OpenBSD base are all written and maintained by the same team with members who have similar knowledge, expectations and experience with regard to software and security, etc.