They exist[0], but aren't cheap (~$1400 US/chip for the base model).
[0] https://www.microchip.com/en-us/products/clock-and-timing/co...
[0] https://www.microchip.com/en-us/products/clock-and-timing/co...
(Don't worry, I know they are considered atomic clocks, I have 2 Rb modules.)