What is the most common reason nearly all stars look faint from Earth?

Vast spatial separation from Earth.

Stars are overwhelmingly located at immense distances, measured in light-years, meaning the light captured is many years old. Due to this colossal spatial separation between Earth and nearly every celestial body other than the Sun, the vast majority of stellar light is spread so thinly across space by the time it reaches our eyes that they appear incredibly faint. Even the Sun, an intrinsically much brighter star than Proxima Centauri, would still appear billions of times brighter than Proxima Centauri even if Proxima Centauri were moved to the Sun's location, highlighting distance as the dominant factor in perceived faintness.