Qfyilyi

It is a common theme in aggadic literature that

• The Avos were incredible tzadikim


• They more-or-less kept the entire Torah, including some derabannans

If the mitzvos were already known and kept, and the full text was not necessary for them to have perfection in their middos, why was Revelation necessary?

(I would guess that a potential answer is that their descendants forgot it, but that also seems strange. See also here.)

1) Revelation at Har Sinai was necessary to make it mandatory. As Mizrachi explains, the reason Avraham waited, until commanded, to perform circumcision is because there is greater merit in fulfilling a commanded mitzvah than an uncommanded one.

2) See the Rashba's explanation / understanding of this idea, that they were spiritually sophisticated and understood and intuited the fundamentals, applying it to their unique circumstances. This is not the same as keeping all the mitzvot.

3) But even according to the common explanation, Chazal said regarding Avraham "that his two kidneys expressed to him chochma like two teachers". This is intuition and knowledge of how to act, in according with Biblical and even Rabbinic law. Without a fixed text and methods by which to understand principles and derive law, one cannot expect subsequent generations to be similarly sophisticated and intuit how to act.