If I put a red billiard ball and a blue billiard ball in a bag, leave them for a while, and then draw one out, I will find I am holding a red ball or a blue ball. Never a purple ball.
At the level of fundamental particles, we know from experimental evidence that electric charge is a discrete quantity and behaves like the colour on the billiard balls, not like a continuous quantity such as temperature. This is shown in experiments such as Millikan's oil drop experiment. Why ? That’s just the way the world works.