Your eyes and a camera work very differently in low light. The Milky Way is usually too dim for your eye’s color-sensitive cone cells, so in dark conditions you mostly rely on rod cells, which are more sensitive to faint light but don’t show much color. That’s why the Milky Way may look grayish or faint to the eye.

A camera can also gather light for much longer than your vision can. Long exposures — and especially stacked exposures — collect far more total light than your eyes can integrate, revealing faint stars, dust lanes, and color.

Light pollution and haze make this much worse. In cities, the sky is too bright to see faint Milky Way detail. Under a very dark, clear, moonless sky, you can see much more structure, and sometimes a little color, but still not as vividly as in photographs.

So the main reasons are: your night vision is poor at color, your eyes can’t accumulate light like a camera can, and sky brightness often hides faint detail.