To make sense of why the past tense is used reinterpret the literal translation this way:

Clouds were hung in the sky.

The result of clouds in the sky is the present state we call “cloudy.”

The words for cloud 구름 shadow 그림자 and shade그늘 share similar characters. Can somebody explain the etymology?

Why only plural?

Could this sentence also be written "하늘에 구름이 있어요" or not?