Why does it rain all over the sky at night? Just because there are stars doesn't mean it must be sunny.

Because different times in an area may be controlled by different weather systems. When the sky is full of stars, it may be located at the high pressure center or ridge, or it may be located before the cold front and after the warm front. Because the weather system is constantly moving and changing, if the front moves behind the cold front, or before the warm front, or the high pressure moves away and the low pressure moves, it may rain. So the weather is constantly changing.