What were the odds that ʻOumuamua passed so close to the Sun?
How could it be found otherwise? It is currently (November 2018) only at about the orbital distance of Mars and cannot be detected by the world's largest telescopes.

Such objects are small and only seen in reflected light - they have to get close to the Sun (and Earth) to be seen. There may well be a substantial population of interstellar rocks passing through the solar system unseen at greater distances.