Code aside, the goals for each project are vastly different. There's nothing to be gained by consolidation.
FreeBSD: I always got the impression this was trying to be full modern UNIX but non-linux
NetBSD: I guess this is for older/less powerful computers based on comments here?
OpenBSD: ???Security???
Dragonfly: a schism over threading, but FreeBSD?
For hardware, can a single device driver be made for all variants of BSD? If so, then I agree.
From what I've seen, the BSD community swaps code around on a regular basis. But they pick and choose what code to use based on their own goals. It seems to work pretty well.