Why can't I sync Photos on my Mac with Photos on my iPhone?
The solution, it turned out, was to quit Photos then relaunch it while holding down the Option key. It then asked me to pick a system photo library and I re-selected the existing system photo library. Then when I launched iTunes, "sync photos" gave me the option of syncing selected albums from Photos, as desired.

I've no clue what went wrong or why this would fix it, but it did.