Avraham as the world's foundation

הרמב"ן ז"ל הקשה למה לא נזכר מקודם בפסוק צדקת אברהם אע"ה רק לך לך כו'

The Ramban asked: why does the verse not mention beforehand the righteousness of Avraham Avinu, but begins only with "Lech Lecha — go for yourself," etc.?

The Ramban is puzzled that the Torah opens Avraham's story with Hashem's command to leave his land, without first telling us of the tzidkus (righteousness) that made him worthy of being chosen.

ונראה כי לבעבור אברהם אע"ה הי' קיום כל העולם כמאמר בהבראם באברהם כו' לכן לא שייך לתלות הדבר בזכות מעשיו כי ודאי קיום העולם הוא הקב"ה רק שזכה אאע"ה להיות הוא הגורם אעפ"כ הכל בעזרת הבורא ית' והבן זה.

And it appears that because the existence of the entire world was for the sake of Avraham Avinu — as Chazal expound on "be-hibaram (when they were created)" as "be-Avraham (through Avraham)" — therefore it is not fitting to attribute the matter to the merit of his deeds; for surely it is the Holy One who sustains the world, only that Avraham Avinu merited to be the cause of it. Even so, it is all with the help of the Creator Yisborach — understand this.

The Sefas Emes answers that Avraham's role was too fundamental to rest on his particular good deeds. The world was created for his sake, so he is the channel through which Hashem sustains all of creation. Yet even being that channel is itself a gift from Hashem, not something Avraham earned by his own actions.

וכמדומה לי שכ' זה או כדומה לזה בספרי מהר"ל ז"ל:

And it seems to me that this, or something similar to it, is written in the works of the Maharal.

The Sefas Emes notes that he believes the Maharal expresses a comparable idea in his seforim.

Summary: The Torah does not preface "Lech Lecha" with praise of Avraham's deeds because Avraham's greatness was not earned by particular acts — the entire world was created for his sake, making him the conduit through which Hashem sustains creation, a role that is itself purely a gift from the Borei.