I’m not sure why that’s still the case, but hopefully that’ll be remedied sooner rather than later. They said shortly after Duo Cam Doorbell’s launch last October that they were shooting to make Wi-Fi Chime available in December:
If you were installing in a wired configuration and wanted to actuate your home’s built-in chime (if you had one), then I’d share that I learned just today that they’ve finally made the Chime Controller available again as a separate purchase, but I still don’t see the Wi-Fi Chime for sale as an accessory.