Because of the permissive MIT license, they can freely incorporate each others codebase, as can any software project, commercial or not, open or closed.

The Linux codebase, on the other hand, is licensed under a copyleft license that only allows its use in open-source projects that themselves only allow their codebase to be used in open-source projects, and so on. Because of this, Linux can incorporate BSD codebases, but not the other way around.

Code aside, the goals for each project are vastly different. There's nothing to be gained by consolidation.

What are the goals/specialization/actual use of each?

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?

Every Linux distro has different goals. But a unified kernel (more or less).

For hardware, can a single device driver be made for all variants of BSD? If so, then I agree.

You aren't going to see OpenBSD share a kernel with anyone - it's too different and makes trade-offs the others won't accept. And NetBSD doesn't need the heavyweight kernel FreeBSD uses.

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.

I know nobody will touch this with a ten foot pole, but from a thousand mile view the BSDs seem like a good candidate for a microkernel.

There's a lot of shared code.