measurements - How is a second measured? And why is it measured that way?

The earth's rotation around the sun isn't exactly 24 hours. It off by some seconds which becomes somewhere around 6 hours per year and 1 day in 4 years(leap year), which brings the question why didn't we modify the measurement of 1 second ever so slightly so as to avoid leap years altogether.

Well how is 1 second measured exactly? Wikipedia says

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. [1]

Well, why is it measured that way? Is there any technical reason like it is easy to measure and can be standardized easily? Or is it really possible to modify the measurement of time?

Answer

why didn't we modify the measurement of 1 second ever so slightly so as to avoid leap years altogether.

The rotation of earth and the revolution of earth around the sun is not at all synchronized. The Earth really rotates 365.24219647 times during each revolution (at 1992, this ratio changes slightly every year, the tropical year gets roughly a half second shorter each century); so even if we fixed the definition of time to the revolution of earth around sun, we will still need a leap year every 4 years (what we wouldn't need would be leap seconds).

Another reason is because precise time measurement would become incomparable. Since the period of rotation of earth (i.e. tropical year) isn't constant, if we used the definition of second to exactly match the period of revolution, then whenever you want to specify a precise duration of time, you'll also have to specify which year that definition of second is taken from, and you'll need a table that records the length of second of each year.

Is there any technical reason ... ?

Yes, because with the proper equipments anyone, anytime can take a caessium-133 atom, put it under the specified condition and measure the same second, and it won't have yearly change like second from earth rotation/revolution would. As far as we know, the frequency of caessium-133 in the 1978 should be the same as the frequency of another caessium-133 in 2049.