Why do kilometers become impractical for interstellar and interplanetary distances?

They result in numbers that are unwieldy, prone to transcription errors, and divorced from human intuition.

The fundamental issue with using kilometers for vast cosmic distances is the sheer scale of space, which leads to extremely large numerical values when expressed in this unit. These enormous numbers, such as the tens of millions or billions required for distances to outer planets, become unwieldy for everyday use in scientific communication. This unwieldiness significantly increases the risk of transcription errors during calculations and reporting. Furthermore, such large numbers are utterly divorced from human intuition, making it difficult for scientists or the public to immediately grasp the comparative magnitude between two distant points. For instance, comparing $400,000,000$ km to $750,000,000$ km is cognitively more demanding than comparing $2.7$ AU to $5.0$ AU.