Why doesn't the Torah explicitly relate Avraham's early life?

Why is Avraham's early life not mentioned in the Torah? That is, the midrash records a lengthy story about his concealment at birth, smashing his father's idols, being brought before Nimrod, his defiance, and so on and so forth. This is a remarkable story. How come we only first hear of Avraham when he is 75 years old?

This is addressed by Ramban, who writes regarding Ur Kasdim and what happened there:

אבל התורה לא תרצה להאריך בדעות עובדי עבודה זרה ולפרש העניין שהיה בינו ובין הכשדים באמונה, כאשר קצרה בעניין דור אנוש וסברתם בעבודה זרה שחדשו:

"But the Torah did not wish to go on at length in the opinions of idol worshipers, and to explain the matter which was between him and the Chaldeans in matters of faith, just as it shortened in the matter of the generation of Enosh, and their position in the idolatry they innovated."

Someone asked me (via email) whether anyone else addresses this issue:

Besides the answer of the Ramban, who else discusses why Avraham's life story from birth to 75 is not in the text of the Torah?

Here was my response, lightly edited:

None that I know of.  But then, I haven't researched the question.

But one might answer that the Torah does not tell us all sorts of details about people.

The Torah does not tell us explicitly about the brothers eating ever min hachai. It does

not tell us about Avraham establishing eruvin, or printing currency, and so on and so 

forth. There is a Biblical narrative with a specific group of details it wants to convey.

Other details were not part of this Divine intent. (Just as there were many neviim and nevuot which did not make it into Tanach.)

Or, to seize upon the story of Nimrod and the fiery furnace, this hangs rather loosely

on "Ur Kasdim" together with details gathered from sefer Daniel. A similar situation

occurred for the midrashim surrounding Pharaoh's dreams. While this might be a case

of maaseh avos siman labanim, my intuition is that (or at the least, one might say)

that these were not necessarily intended as history, but rather homily, or else to stress

certain aspects of Avraham's underlying character. 

(I can add other obvious approaches, such as that Chazal interpreted that they happened, and believed that they happened. But they did not happen, and so the Torah omitted it.

Or, it is in the text of the Torah. It is just encoded. If the Torah explicitly included every detail, it would be much, much larger. And Hashem is concerned for tircha de-tzibura. Leining on Shabbos is long enough, and we are all pining for the chulent. What if krias haTorah were five times longer? And so, most of the content is encoded in shorthand, via textual irregularities to be interpreted.

Or, this way, we won't mess up the Torah Code about Elenin and Nibiru.)

It is interesting that many, many Jewish people actually do think that the story of Avraham and Nimrod is written explicitly in the Torah. Try it out.

How about you, dear reader? Any meforshim you know of that address this? Absent any meforshim, any perspective?