Goes for any light source really, just using a fireplace/bonfire as an example. If you were to look at a bonfire from a distance in the dark, the light seems to stop rather suddenly. But the only way you’re seeing it in the first place is from the light hitting your eyes.
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There are two reasons for this. One has already been mentioned by the other posters, which is the inverse square law.
The second, which has more to do with the appearance of a sudden drop off than anything else, is that the fire itself is an open light source, and as long as the fire is within your field of view your eyes are going to adjust to its brightness, which dims the hell out of everything else by comparison. You get the same effect if you take a lampshade off of a lightbulb. If you stand with your back to fire fora couple of minutes you’ll see that the rest of the area is lit up a lot more than it seemed before, because your eyes have adjusted back to the relative darkness.
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