[URSpider94]

If you go back in history to when the SI units were defined, there would have been no way to measure a Coulomb of electrons, since in nature you'd somehow have to distinguish the electrons you are counting from all the other electrons sitting around. With the invention of single electron counting, we could do it now, but we aren't going to change the SI system a century in. And, the idea was that the seven primary units are both independent (you can not derive one from any of the others) and sufficient to derive all of the other units of measure. Once you have the Ampere, you can define a Coulomb as a relative quantity.