וַיִּרְאוּ כָּל הָעֵדָה כִּי גָוַע אַהֲרֹן
The whole congregation saw that Aaron had expired.
Rashi explains: When they saw Moses and Eleazar coming down, and Aaron did not come down, they said, “Where is Aaron?” He said to them, “He died.” They said,“ Is it possible that the one who stood up against the angel and stopped the plague can be overpowered by the angel of death?” Whereupon Moses asked for mercy, and the ministering angels showed him to them, lying in the bed. They saw [him] and believed.
When R. Chaim Soloveitchik passed away near Warsaw, his son, R. Moshe Soloveitchik, who lived in White Russia some distance away, was informed about his death by reading the newspaper. He immediately traveled to Warsaw to confirm the news, despite the fact that taking such a trip was dangerous, coming at the end of World War 1. R. Hirsh Levinson, the Chofetz Chaim’s son-in-law, inquired why R. Moshe needed to do so: was the newspaper item insufficient evidence to allow R. Moshe to sit shiva without independent confirmation?
R. Moshe answered R. Hirsh that when the prophet Elijah was taken to heaven, his disciples had already known that he was about to die, and indeed they even had a vision of him being taken heavenward. Yet, they still insisted on searching three days for their missing teacher (2 Kings 3). When a rebbe is taken from his students, the students cannot conceive of life continuing without him so they desperately search for him, despite the evidence of his passing.
The same dynamic holds true here. Despite the testimony of Aaron’s death, the people could not accept the bitter reality until they saw it with their own eyes. (Birchas Yitzchak, p. 226-227, Meged Giv’os Olam, Vol. 1, p. 9)