It’s been a long time since a Kilogram was defined as the weight of a particular rock. As science progresses definitions for units of measure get redefined whenever we discover a physical property that’s more stable than what we used before. For example, the original definition of a meter as the length of a particular rod didn’t take into account the fact that it would expand in the summer and contract in the winter (or during the day and at night). These days a meter is defined as the distance light in vacuum travels in 1/299,792,458 of a second, which doesn’t vary with temperature.