Why does 'human skin' not have an article?

The following are acceptable:

1. The human skin owes its color to the presence of particles known as melanin.

2. A human skin owes its color to the presence of particles known as melanin.

Sentence 1. might be used, for example, in the following:

We humans take our skin for granted, but it is a marvelously complex organ in its own right, rivaling other organs

In the context you cite, skin is regarded as a substance - uncountable.
All the others you mentioned are regarded as countable entities.

As a countable object, a human skin would be the entire skin of a human being detached from the body. For example, a bear skin can be used as a rug, but when it's on the living animal it's just skin - bear skin.

1. The snake sheds its skin once a year.
2. cosmetics for sensitive skins.

a. In sentence 1, is 'skin' a countable noun?
b. In sentence 2, even though sensitive skins are not detached from the body (!), why the noun skin was used as a countable noun?

It is very difficult for learners whose language doesn't have articles to fully understand the usage of articles. In

awww it's Science querry nor English, isn't it??

1. The snake sheds its skin once a year.
2. cosmetics for sensitive skins.

a. In sentence 1, is 'skin' a countable noun?
b. In sentence 2, even though sensitive skins are not detached from the body (!), why the noun skin was used as a countable noun?

1. skin is uncountable I'd say. It sheds all of its skin -- all of the skin it has.

There are innumerable situations where articles are or are not used. You just have to learn them through experience, as there are no rules that cover every possible situation. The following are acceptable:

The snake just shed a skin.

There's a skin that the snake shed.

There's the skin that the snake shed.

There's skin that was shed by a snake.

There