Why do you think that is? Because that's what you are USED TO, of course. If you told me you skis are "six foot" long, I would have no clue if they were too long or too short for me. If you told me they are "one eighty", I'd know right away.
If you told me they are 1800 mm long, I'd still know right away. If I told you they are 72 inches, you'd have to think about it for a second. It is more convenient to move a decimal separator than to divide by 12.
You've got it all backwards. I would make that trace 0.15 mm, which is much more convenient than 5.90551181 thousandths of an inch.
Obviously, the inch is more convenient when you choose an example that is a nice, round fraction of an inch.
Your example uses a unit that is ONE THOUSANDTH of an inch. You can't do that. The thousand (and the hundred and the ten) belongs to the metric system. The imperial system uses halves, quarters, eighths and so on. Your trace is roughly 3/512 of an inch if you were to stick to your system.