Mass is the last fundamental unit that is still defined like that.
Previously, meter was also defined as the length of a specific item, kept also at Sevres, in France (since the metric system has been popularized starting from France).
But the issue with mass, is that mass and energy are kind of interchangeable, so that defining a mass unit by using specific atoms is very tricky if you want the definition to be usable everywhere, that is fairly easily reproductible with an experiment method.
The current definition of the meter is not very easy to use, but it fairly accessible and absolutely reliable and reproductible.
The next definition of mass unit, as per the existing answer, is not using the same path, an rely on a fundamental constant and leverage on the meter, that can be defined precisely with a experiment method.