Why is exposure a subjective decision?

An in-camera or external light meter will tell you the settings to use to expose the image "correctly", i.e. for the mid-tones. It is just data - aperture, shutter speed & ISO. 

The photographer then interprets that data to place the image where he/she wants to - overexposed for a high key look, 'normally' exposed, or underexposed (low key look). That is where the creative photographer takes the data he is given & then makes a subjective decision as to where to place the exposure. 

That's my take on it anyhow!

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