FreeBSD, NetBSD, DragonFly and OpenBSD do not share the same kernel...

But they share a common, non-restrictive source license. OpenBSD could have imported 1:1 threading from FreeBSD, which added it earlier.

OpenBSD has a huge anti-NIH sentiment. This can be a good thing, as bringing in code from others = bringing in their bugs as well.

For example, rather than import the quite huge Intel formulated ACPI stack, they have one of the only non-Intel, non-Windows one out there.

See also their reimplementations or forks of SSH, NTP, SMTP, BGP, etc. all done with a security focused mindset.

As a result they also have an 8-year stale (and growing) feature set compared with Linux or FreeBSD. And bringing in used, tested, and maintained code from others brings in their bugfixes as well as their bugs.

If you want the new features, use the systems that have them. OpenBSD would have little point to it if it tried to keep up with the Jones'. It has a unique focus - that's why it's useful to some people.

For many people, the lack of Linux "features" is a feature.

OpenBSD rthreads was started at the roughly same time as the equivalent work in FreeBSD and NetBSD. It took a bit longer, but we got there. And both FreeBSD and NetBSD basically tossed out their first implementation, then rewrote it again. Fun times.