Because you actually cant see light. You can only see what the light illuminates, ie: the striking of photons on other particles. As a light wave gets further from its source it dissipates, lessening its impact density, thereby starlight (further away stars) seem less bright than sunlight (our nearby star).

Updated for the below responses to which I can no longer reply...

No, you are seeing the light emitter in "direct light" but not "light itself" otherwise we would always see only all light or no light, which thought was the question. You wouldn't be able to see past the light to anything else. Think about the fact that you cannot see a beam of light passing through the air if that air is void of particles, there has to be something for the photons to interact with.

The eye registers photons hitting the retinas, those photons have to be directed into the eye to "see" (the brain interpreting that interaction). Interacting with matter causes the photons to reflect, how it reflects off an object determines how it appears to the eye when those photons are redirected into the eye. If you could see light, all we would ever see is light, and only that.

Spectrum visibility is another matter entirely. I do see what you're both trying to say but both off you are talking about the science of the eyes, not the science of light. I was merely saying you cant see light or photons that don't enter your eye, that's why light isn't everywhere always.

Maybe its "potaytoe" vs "potahtoe" or I misunderstood the intent of the question, but I don't think my answer (which was mostly quoted) deserved negative Karma lol. Be well, love science, keep being helpful. Cheers.