A long long time ago the standards for meter's, kilograms, etc was an actual physical object.
Those days are long gone of course. For example, the meter is defined is the length of the path traveled by light in 1/299,792,458 th of a second. Why such an odd number (can fractions be odd?)? Because they retrofitted it to what that the meter is. Rather than go to France and compare my stick to the one under a glass case I can just measure the speed of light. Oh. That sounds hard!
It matters a bit since the weight of what was the standard kilogram did increase over time, though of course not by much. When did the measurements for stuff STOP being based on physical objects and was all done based on constants of the universe?
The answer surprised me:
On Nov 16, 2018 (yes, you read that light) they decided that by May 20, 2019, the Kilogram will be defined in terms of Plank's constant. I have not been able to find out how they will use Plank, maybe they don't know yet (they do and its known -- see the first comment) .With that, there are no more standards based on physical objects. Read about it here.
Why did it take so long? I honestly don't know and I am tossing that question out to my readers. You can leave serious or funny answers, and best if I can't tell which is which!