Why doesn't the God just tell what we have to do in life?

God does indeed tell us what we have to do in life. In Bhagavad Gita God tells humanity exactly how to act in the best possible way for ourselves, nature, and everyone else in the world.

Krishna says:

"Now hear, O son of Prtha, how by practicing yoga in full consciousness of Me, with mind attached to Me, you can know Me in full, free from doubt."

"I shall now declare unto you in full this knowledge, both phenomenal and numinous. This being known, nothing further shall remain for you to know."

--Bhagavad Gita 7.1–2

In fact, Bhagavad Gita is the only book on the planet in which God describes in detail about Himself, the soul, and the world, and how they are meant to inter-relate.

Krishna’s teachings are the pinnacle of philosophy and religion. The philosophy delivered by Krishna in Bhagavad Gita is without a doubt the most complete, consistent, and scientific explanation of reality available anywhere in the world at all times.

Krishna says:

But those who always worship Me with exclusive devotion, meditating on My transcendental form—to them I carry what they lack, and I preserve what they have. (Bg. 9.22)

Those who are devotees of other gods and who worship them with faith actually worship only Me, O son of Kunti, but they do so in a wrong way. (Bg. 9.23)

I am the only enjoyer and master of all sacrifices. Therefore, those who do not recognize My true transcendental nature fall down. (Bg. 9.24)

O son of Bharata, as the sun alone illuminates all this universe, so does the living entity, one within the body, illuminate the entire body by consciousness. (Bg. 13.33)

The sky, due to its subtle nature, does not mix with anything, although it is all-pervading. Similarly, the soul situated in Brahman vision does not mix with the body, though situated in that body. (Bg. 13.34)

Those who see with eyes of knowledge the difference between the body and the knower of the body, and can also understand the process of liberation from bondage in material nature, attain to the supreme goal. (Bg. 13.35)