Why we shiver/tremble/shake while performing some work which requires high accuracy?
If you thread needles often - like a dozen times a day for many days - you will eventually be steady as a rock, because practice improves the process (you will also unconsciously maneuver to steady both hands.) It's not inevitable that all people have involuntary muscle movements with very fine motor skills; it's a matter of ability/practice for most. 

The steadying of larger muscle groups like the upper arms - especially against gravity - is less controllable (e.g. threading a needle with arms fully extended), but not even closely outside the realm of possibility. Think of what great athletes, dancers, craftsmen are able to do that is "impossible" for mere mortals.