Because your brain doesn’t just straight up show you the raw color value sent by your eyes, it tries to estimate the true color of the object based on lighting and context cues.

If you look at a chess board with a gradient shade on it, you brain will tell you the squares are black and white, even tough they are really all completely different shades of grey to your eyes. Depending on the context, literally the same color can appear black or white to you.

All because your brain is trying to be a smartass.