But they are in the same repo because they are tightly coupled. Some of the code is shared between kernel and userspace for example, and stuff is versioned to match, as the BSDs do not have the strict userspace compatibility guarantees that Linux does, so things do change (though there are compat shims).
On the other hand, with the BSDs, you can swap out nearly any userland component for an alternative and it keeps working. While various things share code, it's very rare that one feature depends on an entirely unrelated feature, which is extremely unlike systemd.
