Answer:
Because they are too small
Step-by-step explanation:
The radius of the Earth is approximately 6370 km. This means that distances between farther places in the Earth, e.g. between two cities, is usually measured using the kilometer. Shorter distances are even measured using the metre.
On the contrary, distances in space are much, much greater. For example, the average distance between the Earth and the Sun is approximately 150 million kilometers, which means
. For farther objects, it is even unpractical to measure the distance in million km. For example, the closest star to the Sun, Proxima Centauri, is 4.2 light-years from us (the light-year is the distance covered by the light in one year: 1 light year is equivalent to
).
More distant objects, such as galaxies, are even measured using another unit, the parsec: 1 parsec (1 pc) corresponds to
.