Because Astronomers use light-year instead of kilometers to measure distances in the universe because distances in space are so huge that km would be too large to manage.
A. Putting money into an asset
No-one knows for sure. A popular theory has it that the journalist Lowell Thomas helped spread the myth in his preface to Dale Carnegie's block-buster self-help book How to Win Friends and Influence People. Thomas misquoted the brilliant American psychologist William James as saying that the average person specifically "develops only 10 percent of his latent mental ability." In fact James had referred more vaguely to our "latent mental energy." Others have claimed that Einstein attributed his intellectual giftedness to being able to use more than 10 percent of his brain, but this is itself a myth. Another possible source of the 10 percent myth is neurosurgeon Wilder Penfield's discovery in the 1930s of "silent cortex" - brain areas that appeared to have no function when he stimulated them with electricity. We know today that these areas are functional.
I believe that all of these answers are reasons why engraving can be difficult, except the one that says that many artists struggle with the sound the tool makes while etching on metal.
If they minded it, they wouldn't be doing this job.
