I'm `cvs get`ting as fast as I can to read the rest of sys_fork.c to sate my curiosity, but CVS incredibly slow. I'm spoiled by how git packs the repo.
$ time cvs -d anoncvs@anoncvs1.ca.openbsd.org:/cvs export -rHEAD src/sys/kern/kern_fork.c
U src/sys/kern/kern_fork.c
0m2.73s real 0m0.06s user 0m0.04s system 1. it works
2. they're used to it
3. there isn't enough reason to change
4. lots of infrastructure would need rebuilt if they changed