They just choose not to, for (as I understand it) a combination of reasons. Being averse to relying on new GPL code, being "happy enough" with CVS (also working on OpenCVS), and CVS being a little more than just source control for them due to the likes of CVSync[1].
I have patches in Git. If OpenBSD doesn't want to use it that's fine by me, and I wish them the best of luck. But let's be clear, it's not because we're telling them they can't use it.