macOS is BSD roots on top of Darwin
While true it doesn't answer why OpenBSD is considered more secure by default than Linux. Despite its BSD roots, macOS has had its share of CVEs:
https://www.cvedetails.com/version-list/49/70318/1/Apple-Mac...
That's not specifically OpenBSD, though. The BSD world is not the monolith that it was back in the 1980s.
While true it doesn't answer why OpenBSD is considered more secure by default than Linux. Despite its BSD roots, macOS has had its share of CVEs:
https://www.cvedetails.com/version-list/49/70318/1/Apple-Mac...