Comment by hackingonempty

Symmetricom (now owned by Microchip) released the first Chip Scale Atomic Clocks in 2011. It probably took some time for them to become available to watch designers and for them to be integrated into watch designs.

https://www.nist.gov/noac/success-story-chip-scale-atomic-cl...

I'm guessing the watches sold poorly and the chip packages are still large compared to a watch movement, so no new designs.