Why is the second not defined by length of the day?

2 answers

because

The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

A day is how long the earth takes to rotate once and that in our time is 24 hours [well, 23 hours 56 mins and 4 seconds] and is fixed.

In internet time [beats] it is 1000 beats exactly. Different units give different answers.

One last example - the distance between say, New York & Miami is fixed but can be expressed in Meters or Yards but neither the meter nor yard were defined by that distance.

I hope that makes some sense.
Oh and I don't think this is a Chemistry question.