in astronomy, why does flux decrease with distance but not intensity?
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Intensity of light has two different definitions depending on which branch of physics you are in. It is a bit messy when there are photometry, radiometry, optics, astrometry and thermodynamics that all handle same things but may not use same names for them.
Definition 1: Intensity is power transferred per unit area. Also called flux. This gets smaller as you get further away from the light source as the same power is spread out over larger surface area.
Definition 2: Intensity is radiant power coming out per unit solid angle (solid angle is like normal angle but in 3d instead of 2d). This is property of the light source and so does not change with distance.
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