Question

Why are astronomical distances not measured with standard referent units of distance such as kilometers or miles?

Answer 1

Because astronomical distances are so large (1 ly = 9.5 x 10¹² km)

If your talking in terms of our solar system they do sometimes use kilometers of miles to measure distances, but these distances are so great that is hard to comprehend them.

So instead of using KM's to measure distances Astronomical Units or AU's are used. An AU is simply the average distance between the Earth and the Sun. Because the Earth's orbit around the Sun is an ellipse, the Earth is not always the same distance from the Sun. An AU is equal to ~149,600,000 km. It takes 8 minutes for light to travel from the Sun to the Earth,traveling at the speed of light.

And when you get to measuring distance outside of our solar system the distance between two objects is even bigger so we start to get into using lightyears as a measure of distance. One light year being equal to the distance light travels in a year or 1 light year = 9.4605284 × 10¹² kilometers