The design of power transmission and distribution networks is driven by several physical realities: high voltage is needed to transmit power over long distances with acceptable losses, it is easier and cheaper to transform voltage and switch ultra high power circuits in an AC network than a DC network, all power conversion is lossy, electricity is hazardous and therefore electrical circuits must be insulated and protected adequately to protect public safety.
I've heard this question of "why don't we have some kind of universal power outlet that any load regardless of voltage or current needs can connect to?" before. The answer is the same as in many other fields of engineering: it's not efficient to overbuild every node in an network to handle any possible load when electrical loads are usually well-defined and do not change rapidly. We don't have a universal train that runs in the metro, for freight and for high speed rail not because it's not possible but because that would not be efficient. The same applies here.