The short answer is because our eyes are much more capable and our minds are much more intelligent than our camera's eyes and image processors. Our eyes also don't "freeze" pictures. We watch it as it happens, so things like motion blur and camera shake don't affect our eyes.

A longer answer:

Our eyes have a wide dynamic range that allows us to see more light than a camera can. That means we can see more detail from shadows (dark areas) to highlights (bright areas) than a camera (at the time of writing). This factor alone would contribute to your answer.

For example, when we look at a sunset, we know what we're looking at. Our eyes can see "into" the shadows and highlights far better than a camera can. Our mind sets the "white balance" for the warm sun light. Our eyes are simply amazing and mind even more so.

Our camera's do their very best to adjust and show us the same thing we see with our eyes.

Camera engineers create tools, settings and adjustments that allow you to help the camera capture what you see. Things like scene modes (landscape, portrait, face detection, back lit, beach, snow, and so on), semi-automatic shooting modes (Aperture Priority [Av or A] Shutter Priority [Tv or S]) all do something to help the camera make better decisions. But, they're all automatic modes. If you leave your camera on beach mode, it won't automatically change modes if you begin shooting a car race, or a flower.

In a nut shell, cameras can make poor decisions because they don't know what they're looking at. It's your job to know what your camera is capable of, and how you can help your camera know what it's looking at. This will help you capture photos that more closely match what you see with your eyes.

Skim through your camera manual to learn more about the automatic, semi-automatic and scene modes your camera has to offer. When you feel you've outgrown automatic exposure, or, you still don't capture what you want, learn more about manual exposure, manual white balance, raw file format, raw file processing and so on.

Without a specific example, this is the best answer I could provide. I hope you find it useful.