Well, actually, that’s not wholly true…
Because in the Temple, the Shem Hameforash, the Explicit Name, was indeed uttered by the high priests. A recurrent passage in the Seder HaAvoda, the description of the doings of the High Priest in the Temple on Yom Kipur, (one of the most uplifting parts of the prayer on that holy day), says:
וְהַכּהֲנִים וְהָעָם הָעומְדִים בָּעֲזָרָה כְּשֶׁהָיוּ שׁומְעִים אֶת הַשֵּׁם הַנִּכְבָּד וְהַנּורָא מְפרָשׁ יוצֵא מִפִּי כהֵן גָּדול בִּקְדֻשָּׁה וּבְטָהֳרָה הָיוּ כּורְעִים וּמִשְׁתַּחֲוִים וְנופְלִים עַל פְּנֵיהֶם וְאומְרִים:
בָּרוּךְ שֵׁם כְּבוד מַלְכוּתו לְעולָם וָעֶד:
And when the priests and the People standing at the Temple’s court heard the holy and terrible Name, explicitly uttered by the High Priest in sanctity and purity, they would kneel and bow, and fall prostrate upon their faces and cry: Blessed be the glory of his kingdom’s Name for ever and ever.
But outside the Temple, and, alas, we have no Temple these days, it is forbidden to utter His name as it is written.
And when Moshe, just before commencing his great endeavor of freeing the People of Israel from Egypt, asked God how was he supposed to address Him, HaShem explained (according to Midrash) :
I am not called (=read) as I am written. I am written Yod Hey (short for the holy Name as it is written in the Torah) and I am called Alef Dalet (short for Adonay – the way Jews usually pronounce His name). But in days to come I will be called as I am written.
We learn that uttering His Name as it is written in the Torah is really a thing of the World to Come, and it is now forbidden.
It seems that the Temple was like a “speck” of the World to Come, planted in this world – therefore it was allowed there. (About uttering the Name in the Temple there is more to be said – but about that, perhaps another time…)
But why, finally, is utterance forbidden?
This has to do with what His holy Name signifies.
More accurately, it does not signify, it is.
The Name of G-d is perfection, infinity, completeness. Therefore, in a world like ours, where completeness is only to be searched for in the depth of Existence, and deep inside our hearts, his Name may be only mused internally, in the mind, not uttered.
Because to utter is to bring forth.
When external existence is complete, then His Name will come out to the open and become explicit as well. There could be no discord between Existence and the Name. They have to fit exactly, because it is His Name that holds everything in existence.
Discord in that sense means – falsehood, and He is Truth.
So, only when the People of Israel are all redeemed, and returned to their Land, safe from foes, the Temple built, all of Israel are faithful to HaShem through practicing the Mitzvot and studying the Torah, when all gentiles practice their seven Mitzvot, when the world is free from all sickness and plague and war, when all these things which are but the manifestation of the Kingdom of God come to pass, only then will His Name be uttered as it is written, because Existence itself reveals it.