I'm guessing the developer tasked with implementing this didn't have access to any other device-unique state, and getting the hardware team to have a new unpredictable value flashed onto each device was impractical.
Years ago I read about a similar predictability for ISP-supplied routers that used the MAC as seed for the default WPA key and the SSID. Once someone decoded the algorithm it was trivial to access many home networks.
At the end of the day, I believe it's cheaper to flash the same firmware image on all of the boards and differentiate them during the first boot or even at runtime like in this case.
