The Tathagata "has not uttered even a word, nor will he ever utter; for not-speaking is the Buddha's speaking."
For two reasons:
They are the truth of self-realisation and an eternally-abiding reality...[The truth of self-realisation has] has been realised by the Tathagatas, that is my own realisation, in which there is neither decreasing nor increasing; for the realm of self-realisation is free from words and discriminations, having nothing to do with dualistic terminology...
What is meant by an eternally-abiding reality? The ancient road of reality, Mahamati, has been here all the time, like gold, silver, or pearl preserved in the mine, Mahamati; the Dharmadhatu abides foreover, whether the Tathagata appears in the world or not; as the Tathagata eternally abides so does the reason (dharmata) of all things; reality foreover abides, reality keeps its order, like the roads in an ancient city...
[W]hat has been realised by myself and other Tathagatas is this reality, the eternally-abiding reality (sthitita), the self-regulating reality (niyamata), the suchness of things (tathata), the realness of things (bhutata), the truth itself (satyata). For this reason, Mahamati, it is stated by me that from the night of the Tathagata's Enlightenment till the night of his entrance into Nirvana, he has not in the meantime uttered, nor ever will utter, one word. So it is said:
From the night of Enlightenment till that of Nirvana, I have not in the meantime made any proclamation whatever.
A footnote to the last sentence notes that "The Zen masters frequently refer to this important declaration."
My own teacher cautioned me when I first visited him that "There is no teacher. Bodhidharma is the teacher."
That is, the Dharma and its practice and realization are there no matter what. Even this "Word Honored One" cannot "talk out" the realization. It's there, and depends not on any one person.
So if you ever wondered why Zen teachers who really teach Zen are the way they are, they are taking a cue from here. Which of course, is no cue at all.