Generating a perfectly homogenous 4C over an environment large enough to contain a kilogram of any useful substance is going to be incredibly difficult and mearusing it even moreso. There's a reason every unit of measurement aside from mass is derived from an electrical property; we can measure electrical properties down to ridiculous precision, measurements small enough that all measurement uncertainty is removed.

Werent they trying to count the atoms in a perfect sphere of silicon?

That is good to know.

You could do that, but if you're trying to set standards, you have to take into account your ability to measure these things reliably and accurately. (It doesn't do any good to set standards that nobody can conform to because their equipment is never sensitive enough to use them.) Can you measure a liter more accurately than you can measure a kilogram of some reference material?

I always thought it was amusing that the basic definition of the liter was taken at 4C, but that lab equipment was calibrated at 20C...

I think that's because 4C is the point where water is the densest, whereas 20C is a comfortable room temperature.

Interestingly enough it started out as being defined as 1 dm³ in the 18th century, then in the early 20th century was defined through water at highest density, then in the 60s back to 1 dm³.