According to this article,
/data ... Wiping this partition essentially performs a factory reset on your device, restoring it to the way it was when you first booted it, or the way it was after the last official or custom ROM installation. When you perform a wipe data/factory reset from recovery, it is this partition that you are wiping.
What about /boot, /system, and /recovery, and /cache?
If Factory Reset wipes only /data, then I wish it were called something else, because if you've rooted your phone, it will still be left with whatever custom ROM you flashed even after a Factory Reset.
On the other hand, if Factory Reset does indeed wipe /boot, /system, and /recovery as well, especially /boot, what does it restore those from? Do all phones come with stock images in a real internal ROM somewhere that it can re-flash those images from?
I can somehow imagine manufacturers not wanting be burdened with the extra cost that such a spec for hardware-recoverable images might have cost them, so it got left out of the standard and left out of most phones, which is why it's so easy to brick them. So, instead, they give you the option to wipe /data and /cache only, and call that a "Factory Reset" because you're not supposed to be foolish enough to root your phone in the first place according to their way of thinking.
Since rooting my Android and reading hundreds of posts, I've found that many of the terms I learned and accepted as a Computer Science grad student in the 80's don't quite mean the same thing anymore so I have to verify what everything really means before I do something.
Comments to this post answer the first part of my question.
But they don't really answer the second part of my question which is really a complaint: Why call it "Factory Reset" if it doesn't really restore your phone to it's stock factory state?
At this point, the question is merely rhetorical. I think I already answered this in my question, unless someone can correct me.