Mounting Instruments with Lock Washers???

Now, I have mounted many instruments with their accompanying brass
screws and never used lock washers. Then a friend asked why not? I
opened my mouth to spout the obvious answer, but no sound came out. I
realized that I have never used lock washers, never seen them used,
and have never seen a loose instrument. Hmmmmm.

- Lock washers are used everywhere when vibration is an issue.
- An airplane is certainly a vibration prone environment.

So I hazarded these ideas;

- The screws, and nut, are (non-magnetic) brass and somehow brass
stays "stuck" better because brass is softer than steel.
- A brass lock washer wouldn't keep its "bite" as it would flatten
over time.
- I don't have any more good ideas.

So, why is it that we never use lock washers when mounting
instruments? And why don't the instruments vibrate loose? Please
keep the answer this side of a physics lecture.

Thanks, John