Why is it kids
Due to the issue the original poster had.
But...still why would you?
As Avorne said, I have been explaining it quite a bit (probably repeating myself too), but here goes again:
Most often, people that do cut themselves are so depressed and feeling so much mental pain for various reasons, that they can't cope with it in any other way. They need to make the pain physical in some way, and the most commonly talked about would be cutting.
It still hurts to cut yourself, just as it does to everyone else, but the following effects and feelings of safety and a pain being easier to handle (as well as the rush of chemicals the body responds to pain with) can often be enough for a person to continue this habit.
The pain is a rather specific response if the person is looking for some kind of punishment, however, but the safety of that pain is still there as well. Knowing that you got what you deserved, for example.
They should like see a shrink or something like that...
They should, but usually the issues behind cutting are unresolved, even to the individual. You can't get help from anyone, if you don't want to talk about it, and putting something as abstract as mental pain into words is something many people fail.
In the case that the individuals are depressed, they also lack the motivation to actually make a change themselves, and they would need someone else to take charge and probably drag them along, which rarely if ever happens.
However I never resorted to cutting myself, although I have strongly considered it.
Probably a good thing you didn't.
But at the same time, someone who cuts his/herself would also take the risk of sinking deeper into a depression due to the physical pain they're inflicting on themselves.
That would probably not happen, since the cutting is what keeps them afloat in the first place.
Then again, a deeper depression would probably lead to them losing the motivation to actually cut themselves anyway, and the parents would probably notice and be able to help at that point, unless they are truly blind towards their children.