This isn’t really a question about light, it is a question about geometry.
Stars emit a certain amount of light in every direction. You can model the amount of light as being spread out over the surface of a sphere with a radius of your distance from the star. The surface area of a sphere can be calculated by multiplying 4 times pi times the radius squared.
As the radius increases so does the surface area of the sphere. The same amount of light then is spread out over a larger area, meaning the light in a given area reduces.
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