What a great question for preparing for Shavuos! I think there are many answers, but I think the best answer is two-fold:
(1) G-d cannot reveal Himself yet because we are not spiritually prepared. Long ago, at Mount Sinai, G-d actually spoke to the Jewish people when He gave them the Decalogue. It was more than they could handle. Our ancestors told Moshe, "speak to us, and we will listen, but do not let G-d speak with us, lest we die." Exodus 20:16; see also Deut. 5:20-24 (Moshe's summary of the event). As high in spirituality as that generaton was, it was still overwhelmed by the Holiness of G-d that they experienced at that moment, and deemed themselves unworthy. Ramban's commentary to that verse offers the point that G-d's personal revelation to the Jewish people posed a great challenge to them and the concept of preparing ourselves to receive the Divine revelation remains a challenge to us today; a challenge that requires us to achieve higher levels of holiness.
(2) G-d appears to be hidden because He wants us to seek Him out. Isaiah 55:6 states, "Seek Hashem where He is found and call Him when He is near." The Targum Yonasan paraphrases: "Seek fear of Hashem while you are still alive, pray to Him while you still can seek Him and while you can still call Him, not after death when it is too late." This is closely related to the first point -- seeking G-d requires that we elevate ourselves spirtually, repent, humble yourself before Him, love Him and fear His wrath. Rav Nachman (the Tanna) held that the opportunity to seek G-d in this manner to be during the 10 days of Repentence between Rosh Hashanah and Yom Kippur. Yevamos 49b.
I believe that when one prepares himself to meet G-d, and seeks Him in prayer -- especially during the High Holy Days -- G-d's existence becomes increasingly real and more and more apparent in nature, in events affecting our lives, and in the eyes of our teachers. And when I go to the beis midrash on Shavuos night, hearing the cacaphony of the bachurim arguing over pages of Gemara, I think I can make out the Voice of Hashem; and then it is not scary at all.