Why are comets from the Oort cloud difficult to observe before they enter the inner solar system?

Comets originating from the Oort cloud are difficult to observe before they enter the inner solar system primarily because of their extreme distance from the Sun. The Oort cloud is a vast, roughly spherical reservoir of icy bodies located at distances typically exceeding 1,000 times that of Neptune's orbit, with the region where new comets originate extending between 20,000 and 50,000 astronomical units (AU) from the Sun.1

At these immense distances, these small, icy bodies are far too faint to be detected directly. They only become observable once gravitational perturbations—caused by passing stars, giant molecular clouds, or the galactic tide—dislodge them from their distant orbits and send them inward toward the Sun. As they approach the inner solar system, the Sun's heat causes their volatile ices to sublimate, forming a bright coma and tails that make them visible to astronomers. Consequently, by the time these comets are first detected, they are already well on their way into the planetary region.123