Why do we need light to see in the dark? Why can't we see in the dark?

"Dark" is just the absence of light... light is made of photons, emanating or reflecting off of objects - basically, the information about those objects - informing us of their presence... a lack of light from an object means you couldn't distinguish the object from the background - so... you wouldn't know it's there...

No creature or machine can see well in actual dark without a light - we see by photons hitting our retina. No light, no photons, we see nothing. IR cameras and a few animals (pit viper) can "see" in thermal infra-red, i.e. they can see warm bodies in complete darkness by their own blackbody radiation, but the wavelength is longer than visible light so everything is blurry.
We can see pretty well in moonlight or starlight once our eyes have adapted to the dark (takes many minutes), but we can only see in black-and-white, and the bit of our retina where we are "looking" only has colour receptors, so we see better to the side in the dark. That's what we evolved for.

We can see in the dark, but our eyes are designed for working best in the daylight. As humans evolved over millions of years, as hunters and gatherers, it was more beneficial to have excellent daytime vision for daytime hunting. We have excellent color vision, which some animals do not have and that would be useless at night.