Everything is based on everything else.
And although is might be because of not wanting to change,
it is more likely because of economy of implimentation.
Why recreate the wheel?
But this will be true for ANYTHING
even the most modern and sophisicated standards of measurement are based on an agreement of parties to use that standard.
If aliens came down here, they would be asking WHY are you using that scale?
http://www.remote-control.net/convert/tables/general/
Why a second?
Or a meter?
Or a liter?
For example, why is the second based on it's standards?
You can see in the link above there are four different ways of measuring a second.
But let's say you pick seconds as a portion of minutes,
then it's based on...
"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 cesium 133 atom"
http://physics.nist.gov/cuu/Units/current.html
and that's officially derived by only one country's standards
http://www.nist.gov/
BTW, this link is fascinating...look at how many different measurements there are, ways of calculating them
and the implications of each.
Some might save fractions of a penny, that add up to billions in the long run
maybe on a national or global scale.
I love this kind of shit.
How one thing affects another, and so on.