What do turn indicators indicate?

Why are there people who don't use their car turn indicators? It's such a simple thing to do - just flip the thing up or down. Then the vehicles behind and in front know you're going to turn. Also the pedestrians. Watching at a corner I cross a lot, I see that most cars do use the indicator. But why doesn't everyone? Some ideas:
1. Just don't think about - no one ever told them it was important
2. Lazy (you've got to be really lazy to not flick the thing up or down)
3. Already got the steering wheel and (fill in the blank - coffee, cell phone, etc.) in the other hand
4. Think it doesn't matter because no one's around
5. Think it doesn't matter because they're in the left turn only lane, so of course they're turning left (is that ok?)
6. Want to keep others guessing.
7. Just rebelling against all rules

So, do you have some other reasons?

I think people should just be trained to do this when they learn to drive, so it becomes completely automatic for any sort of turn or lane change.

So does it mean anything when someone doesn't use it? Are non-indicators just more self absorbed than others? Do they have more trouble thinking about how their actions affect others? It's so easy to do and such a civil thing to do - saying "just letting you know I'm turning."

Anyway, watch when you're at a corner and see what percent use their turn indicators. Will it be different for different cities? Different countries?