Is bicycle Helmet really effective? Sorry  I am new in bicycling , but I think  the head is least like to be in contact with any other things during crash normally down from head rather the head might come in contact in other vectors . So I am curious how effective bicycle helmates are .
Bicycle helmet is to protect the head, which has the most important organ in human. You could still live a normal life even with broken/ lost limb(s). But a head injury could change your life forever. A helmet is like a wall between life and death. The priority goes by protecting the most important thing to you first (i.e. your life), not by preventing the most common injury. 

Having a priority list, you could fulfil the ' safety pyramid' for extra protection, e.g. helmet, arm/leg guard, gloves, boots. However, it is impractical to get a full body armour for just cycling. The risk of body injury that could lead to life and death situation whilst cycling is rather small. Thus, this is often ignored.

As Frisbee also mention, the brain is also a very sensitive organ. Yet, a seemingly  'minor' damage to the brain is often lethal. Therefore, the head needs protection, albeit a small possibility.

I have witnessed two head injury accidents (while cycling) in my life. One happened a few building block away from my house. It was a classic cornering accident type. The person lost balance (clipped the pavement), and hit her head directly to the pavement (no helmet). She unfortunately pronounced dead at the scene.

The second incident happened on the street. The person was riding in a freezing condition. It is with such unfortune that the person lost control on a straight road and hit his head on the raised curb. He is fiancé of my wife's friend. He is still in coma at this moment (since 3 years ago).

Needless to say, your argument is flawed. It is similar to a statement that: 'I have never been in an accident that an airbag is activated, so is airbag redundant?'