Because they are very far away.
That happens when they are so far away that you can't distinguish the individual stars. Also, to appear as "clouds", there must be a large amount of stars together.
Billions of stars appear as big patches of light in the night sky due to their distance from Earth. From our perspective, the light from individual stars merges together to create these patches, known as galaxies or star clusters. These collections of stars appear as cohesive shapes due to their sheer number and the vast distances between them and us.
That happens when the individual stars are too faint, and too close together, to be seen as individual stars.
Those stars are too far away for us to see them individually, but their combined light makes part of the night sky look brighter.
Clouds appear black when they are thick and dense, blocking out sunlight and making them appear dark. This can happen when there is a lot of moisture in the air, causing the clouds to absorb more light and appear black.
Altocumulus clouds are mid-level clouds that form between 6,500 to 20,000 feet above the ground. They appear as white or gray patches or layers in the sky and are often associated with fair weather, although they can sometimes signal changes in the weather. Altocumulus clouds are made up of water droplets and can occasionally produce light precipitation.
Layered but puffy clouds that occur at middle and lower elevations are called stratocumulus clouds. These clouds typically appear as a combination of layers and patches, often with a white or gray color, and can cover large areas of the sky. They usually indicate stable atmospheric conditions and can produce light precipitation. Stratocumulus clouds form when warm air rises and cools, leading to condensation.
That happens when the individual stars are too faint to se seen individually. It is also because they are so far away from Earth but quite close to each other. It's like seeing two lights which are far away from you. They appear as one big light instead of two separate ones.. It is the same principle with stars.
stratus clouds. They are low-level clouds that appear in a continuous layer and bring overcast conditions. Stratus clouds can sometimes produce light precipitation.
Venus
Light reflected back from the water droplets scatters all the colours of the rainbow - making the clouds appear white.
No clouds always appear white. It is only night because the Earth is rotating and the Sun is not shining light on that part of the Earth.