Earlier quoted context omitted.
The second is defined as the time it takes for two hyperfine levels of the ground state of the 133 Ce atom to make 9,192,631,770 transitions. How is that not special?
Because that definition was grandfathered in such that it matched the previous definition of a second up to the previously specified precision. You've just moved the arbitrary part to the number 9,192,631,770. The same thing was done for the meter (defined such that the speed of light is precisely 299,792,458 m/s), ampere (current which produces 2 × 10−7 N/m of force between two parallel conductors). There's plans to…
Hate to be a stickler, but: "...conductors of negligible cross-section and of infinite length." It's still one of my least favorite SI definitions.