Can't use Linux distribution tools like apt/yum unless you package the update contents as rpm/dpkg files and distribute apt/yum with the base system. Neither of those is happening in OpenBSD, or likely any BSD. (I can go into more detail on why if you're curious.)

In some ways, the BSDs are just catching up to Linux in terms of update distribution mechanism. They all have a history of being self-hosting compile-from-source systems, and some greybeards are still pretty attached to that idea.

As someone who discovered Linux first and happily used it for many years before discovering the BSDs, I think there's a lot Linux gets right that BSDs will or should adopt to be relevant. (There are also many things the BSDs get right, of course.) This is a good example of that.