Because these are two different quantities: illuminance on the subject, and image irradiance at the sensor.

When you move the light closer, the subject receives more light per unit area. That follows the inverse square law, so the subject itself becomes brighter.

When you move the camera farther away, each point on the subject does send less light to the lens, also by an inverse square relationship. But the subject’s image also becomes smaller on the sensor by the same square factor. So the same total light is concentrated into a proportionally smaller image area, leaving the exposure per unit sensor area essentially unchanged.

Put another way: camera distance changes magnification, not subject brightness. The inverse-square loss from subject to lens is compensated by the reduced image size.

This is also why a subject framed the same way at the same f-number gives similar exposure, even if you step back and use a longer focal length. The larger entrance pupil of the longer lens compensates for the increased distance.

So:

• light-to-subject distance changes how bright the subject is
• subject-to-camera distance mainly changes how large the subject appears