one god can have all these attributes. but why polytheism exist? don't many gods have part in nature?
He (or she) could. But not everyone sees God in the same way. In Hinduism there's a verse from the Rig Veda that translates as "There is one Truth the sages call by many names", which has a follow on saying that "God shows himself in a way meaningful to the person". To limited human minds it might be easier to see God in different "parts". There are also degrees of polytheism. It's not "either or". Gods could be manifestations of one God; they could be archetypes; they could be distinct beings that work together; or any number of other views.