The unit V/V

It's about what was a measurable quantity in the late 19th century. Counting ~10¹⁹ electrons would take a long time, but it's "straight-forward" to measure the force two wires exert on each other.

Also, consider that electric current was well-known and widely-studied for many years before the existence of electrons was known and their charge was measured. I don't know a date for the first observation of electric current, but Ohm's law was published in 1827, while the electron charge wasn't measured until 1908.

Since they were first established, we've changed our choice of fundamental units very little, and only as improved measurement technology has come along. At the moment it's still considered easier to measure the force on parallel wires than to count quintillions of electrons, so we still consider the ampere a fundamental unit and the coulomb a derived unit, defined as an ampere-second.

Simply because a watt is a measure of work done. The real part of V*A is watts. There are exact equivalents in mechanical systems. \$W=\dfrac{J}{s}=\dfrac{N*m}{s}\$ so \${N*m}\$ here is Force that does work through a distance but it can also be a measure of Torque over a distance and that is static(non moving). One is the potential to do work, one is the work itself.

Using watts for reactive power would be wrong because reactive power is stored power and not capable of doing work.

Using VAr - r for reactive is just short hand for the purely imaginary part, instead of using i or j (for you physicists out there).